Now that you know the 'why' of the problem, let's look at the solutions to the problem:
There are actually TWO solutions that I use now, and my 'sometimes' favorite one for summer is 'Bra Band Suspenders' below, then after it is a better solution that I wear year round. It's simple in theory... just sew a suspender clip to either end of a 3 inch piece of elastic and clip one to your bra band (usually just under the armpit side of the cup, but wherever you feel it riding up the most) and clip the other end to the top of your jeans, or skirt, or undies. Make one for either side of your bra to keep it in balance. You can bend, twist, stretch, and your bra will stay in place MUCH better! For people with single side mastectomy who also have this problem, you maybe can just wear one. Here's a photo of how a pair should look when you finish:
I looked all over, in local stores and on the web, usually spending around $5 or more for a package with just two clips ($10 for four, so I could make a pair!) and finding that most of the rectangular ones readily available wouldn't hold on all day. Finally, I found these round ones online that gripped well, but the original ones I found were too wide at the ring and I found wide elastic annoying.
Eventually, after much testing, I found that round ones that are about 7/8th's inch wide and have a ring for the elastic that's 3/4" wide, worked just right, if you can find them. If not, use any that you can find. When you look for these, they are sometimes called 'suspender clips' and sometimes called 'pacifier clips', depending on what people are buying them for. No one actually calles them 'bra band suspender clips' except for me, so don't bother asking. :^)
Bra strap elastic that's plush on one side, kept them feeling good, so look for that if you can find it. Regular elastic can pinch, I don't recommend it unless you can't find anything else. (You could actually make a soft flannel tube to cover your elastic if you can't find a softer elastic.)
Flesh color elastic is the best, because it doesn't show through your clothes like white or black elastic can. The silver color ends haven't been a problem for me, but if you worry they might show when wearing very thin light color fabrics, you may want to use a beige fingernail polish to paint the surface. So far, I've not had them show through anything so I've left mine plain.
When you sew your elastic onto the clips, be sure to put the plushest side so that it'll be next to your skin, and fold the ends of the elastic over to sew on the same side as the round smooth covers (away from your body). That may seem wrong to you, but you don't want the ends rubbing on your skin and no one will see them anyway. Your comfort comes first! Be sure to use a little 'Fray Check' (it's a clear glue sold at fabric stores) on the ends of the elastic after you stitch it down, so that it won't leave strings or fray. Plush elastic tends to eventually fray a bit if you don't... not much, but you should still do this step to prevent it happening.
Here's another solution that I've used for several years successfully, and it has the added benefit of slimming your waistline a bit as well as holding down your bra band. :^)
This is very simple to do and takes very little time or skill. You're going to add a 'tube top shape' onto the bottom of your bra that will extend down over your tummy and will hold on snugly, not allowing your bra band to ride up anymore! This will end up looking like one of those body-shaper things, but DO NOT EVER WEAR THOSE after mastectomy, they are positively NOT made for mastectomy use. They are made to squish breast fat into shape at the top section and now all it would do is squish your breast forms firmly into your scars, VERY painful, and potentially dangerous if it impedes the flow of lymphatic fluid, not worth the risk. (That's my warning, I'll get off my soapbox now.)
Above, you'll see the adaptation we're going to make to your bra. In this case, it's the 'Almost U' mastectomy bra with the ultra wide elastic band. (It's the most comfortable bra I've ever found, and yet it STILL rides up into the scars!) I've used this same 'fix' with several different styles and brands, and so far it's worked equally well with all. If you don't think you can do this project as I've described it below, you might try getting a waistline shaper in a larger size than you'd normally buy so that it's comfortable, and stitching it to the bottom of your bra, or sew on snaps or hooks to attach it. If you're ready to give this a try by making your own, let's get started.
Go to your local fabric shop and ask to see the swimsuit fabric. It comes in a range of colors and is made of something called 'Lycra' or 'Spandex'. You probably won't find it at Walmart in the fabric section, you'll have to go to a place like Hancock's or JoAnn's or any other larger fabric store, or you can order it online.
Optimally, you probably should look for a color as close to your skin color as you can, but get whatever you like since it's not going to be seen. I often use beige, black, or white, same as my most often used bra colors. Be sure to get actual swimsuit fabric, not swimsuit lining, which also comes in beige but is a little too thin for this purpose.
Swimsuit fabric is pretty expensive, so just get 1/3 yard (that's 12 inches, for those of you who don't know, lol...) The fabric comes off the bolt in widths of up to 60 inches, so you don't need to care about how wide it is at this point, the 12 inches should be enough to go from the bottom of your bra band down to below your waist. Swimsuit fabric stretches a LOT, so don't worry, it'll still stretch down and cover your abdomen when you put it on, even if you're very tall, like me. While you're at the fabric store, get a spool of a similar color Dual Duty Polyester thread.
PLEASE NOTE: In my photos below, I wanted my 'tube' to not only keep the bra band from riding up, I also wanted it to act as a waistline-shaper, so I cut TWO 12" sections from 60" wide fabric. The fabric yardage was folded in half on the table here, so after I cut it each piece with it doubled, I had to unfold the two separate sections and lay one over the other so that I'd still have a 'matte side' touching my skin to grip better and a 'shiny side' to the outside so that my clothes would skim over it better and not hang up on the matte side. I pinned them together all around the long edges. From here on, you're going to pretend the two layers are ONE layer. Just ignore all those pins on the long edges of the first few photos if you're using only one layer of fabric. Since this was photographed, I'm now almost always using a double layer ONLY on the front of my tube, as it doesn't feel constrictive, but it does still control my tummy somewhat. :^)
If you're thin and shapely (and I am SO jealous of you!), or for normal daily wear, just use one layer because all you need is for it to hold your bra band down. However, if you've got a muffin top, you'll positively revel in how good your shapeliest jeans suddenly look without that muffin top hanging over. :^) Just thought I'd mention it. I'd also like to mention that using two layers isn't as comfortable as one layer. Not nearly. Most of the time when I wear this sort of 'fix', it's with one layer or a double layer sewn onto only the front half of the tube. I reserve just a couple of bras with the two layers of stretch fabric for those days when my fat middle is just TOO bothersome!
Sew the short ends of the fabric rectangle together, so that you now have a tube. I recommend setting a sewing machine at a stitch length of 12 stitches per inch and the widest zig-zag stitch. Set your machine to a 3-step stretch stitch as seen in the photo below. If your machine uses cams, the design on it will look like the photo, or you may have a dial to set it to this stitch. This lets your seam stretch with the fabric more without breaking the threads. Almost all sewing machines with different stitches have a 3-step zig-zag built in, though you may have not realized it before. If your machine is older and doesn't have it, you can get by with standard zig-zag.
Bet you think I've missed something here, right? Here's another 'AHA' moment for you. :^) I'm smiling as I write this, because it was a real 'find' for me. Did you know that from seam to seam, across the back of your bra, it generally has elastic so that your bra band can stretch enough to let you step into the 'neck' of your closed bra and pull your bra up by the straps all the way up over your hips and keep pulling it up while sliding your arms in? It's the same as what you do when you're putting on a one-piece swimsuit.
WAY easier and FASTER than putting your bra on backwards with your bra cups weighed down with your silicone breast forms, hooking it together, then having your breast forms flop around on your hips as you struggle and pull to get the bra turned around your body, then fight it to get it up into place. You're welcome. :^)
Once you try this a time or two, you will NEVER want to put on an open bra gain! My hips are WAY bigger than my bra band size, (I mean WAAAY bigger, ahem...) and I still have bras that I've used for over 3 years, without the stretch being lost one bit from doing this. It's ever so much more graceful to put on OR take off your bra this way. No, I am NOT kidding. Go on, go rip off your clothes and try it right now, even before you make your tube, you'll see. :^) No, really, go on... I won't laugh... I'll just sit here and check my email....
You're back already? And smiling? Now you may ask, "Why has no one ever mentioned this to me before?" Not a clue. It just seems to be such a foreign thing to people who haven't done it, that even some bra fitters I've mentioned it to have just stared at me blankly for a moment. I could almost SEE their minds calculating their bra band size and hip size and saying 'NO WAY!'.
If you SEW with bra-band-type elastics you already know that some elastic will stretch twice it's length or more, so think about it... add 50% to your bra band size since only the back half of your bra band may stretch. (As an easy math example, if your bra band is 40", you'd add another 20" to that, which makes 60" that your bra band would reach when stretched to the fullest... figure this out using your own measurements... is the number you get smaller than your hip size? No, it's larger, right? Alright then! It generally works out the same way, no matter how large or small you are. There ya go, AHA!
Here's a little tip though...if you have any problem pulling up your bra (with or without the tube added) up over your hips, just put your undies on first, before your bra. Seems obvious maybe...but I thought I'd mention it. Undies just help the bra band elastic glide over your hips faster than over bare skin. Then, pull the undies down a bit and pull the lower edge of your 'bra-tube' down over your abdomen. The 'dull' side of the fabric will grab on to your skin and stay there when you pull your undies back up over the bottom of it. I've never had the bra-tube ride up, but then, I usually wear slacks or jeans. If you wear a dress all day, bending and twisting to reach for things, your mileage may vary.
There, wasn't that easier than you thought? :^)