Great serving is about having the ability to select the right serve for the right situation. There is a popular idea that the first serve is just about hitting it as hard as you can - even though this can be relatively easy to deal with, it requires more effort, and does not go in as often - while the second is a little pat that you hope they don't smash back. It is better to consider that you have two serves. And with a variety of spins, speeds and placements at your disposal, your job is to work out each time what it is that you're trying to do.
Avoid becoming predictable
Broadly speaking, on each side of the court, you can aim your serve at the forehand, the backhand, or into the body of your opponent. And you can choose to apply varying amounts of pace, kick, slice or topspin. But you can't be a jack of all trades, so there are always going to be a few core serves that you feel most comfortable with, and you are going to use these most of the time.
However, the better players will always adapt their strategy according to how the game is progressing. For example, if you usually send every first serve to the backhand but your opponent is returning them with ease, perhaps you should switch to plan B. Or, if at 30-30 they are getting a bit nervous and have missed the last few forehands, perhaps you should consider serving one out wide.
You will also need to vary your serving strategy in order to throw your opponent off the scent. This will make your best serves more effective by giving them something else to think about.
When to gamble
If you have got yourself three breaks up and are serving at 5-0 - or perhaps if you are a long, long way behind - you could seize the opportunity to try some serves that you haven't practised as much. Equally, if your opponent is whacking back your most consistent serve every time, you might well think: what choice have I got?
Be cruel
Remember: tennis is a ruthless sport. The great servers are amazingly good at picking the right serve to upset people. It's about assessing what they're not very good at, and then giving them the opportunity to look bad. Andy Murray is particularly skilled at making his opponent feel awful by putting pressure on them at certain times.
Think about when to throw in that slow slice serve to the forehand that once they would have smacked away with lots of confidence, but which now puts pressure on them not to make an unforced error. If your opponent is nervous, they would probably prefer a fast serve so all they have to do is put their racket in the way. This is why a lot of club players have trouble playing someone who just consistently pushes the ball back into court.
Now try this ...
To improve your serving strategy, watch as much tennis as possible, paying attention to all the score situations that you might find yourself in, and especially those that you've been struggling in recently. Work out what your game style is, see which players have a similar style, and examine what their serving patterns are. How do they begin a match? How do they serve a set out at 6-5 up? Are they just rolling balls in and relying on their groundstrokes? Or are they trying to twist the knife a bit, to not only hold serve, but lay the foundations for the next set in the process?
Needless to say, at some stage you are going to have to get out there with a dozen or so cans of balls. Pour out the balls and place the cans as targets in the opposite service boxes. On your wide serves, when you would be trying to take your opponent off the side of the court, set a can down just inside the sideline, about a metre and a half from the net. You want the ball to land as close to the net as possible, because that will mean it has a sharper angle off the side. Now try to hit that can. After this, practise by putting another can where you want the second bounce to be to help you swing the ball out wide. After all, your opponent will have to play the ball about a metre ahead of that point.
To practise the rising topspin or flattish serve you would use to avoid being attacked, try taking it from inside the court, wherever you need to be so that you can hit a ball so that it is still rising as it crosses the baseline. Now take a step back and see if you can do it again. Then another step, until you're doing it from your own baseline. This time, place a can deep into their service box, far from the net, and look for where it has its second bounce. If you can increase the distance between the two bounces, your serve is probably going faster. If you're practising with a partner, ask them to tell you how high it rises as it crosses the baseline.
What am I doing wrong?
Even if you find one serve that is consistently defeating your opponent, do not give them too many in a row. You need to be sensitive to the difference between relentlessly exploiting someone's weakness and turning into their practice partner. After all, if you had a particular weakness in your return of serve, you'd ask a coach to hit to it continually until it stopped being a weakness. So only continue with a successful serve as long as your opponent lacks confidence. You can often tell, because tennis players can be quite emotional.