Aces (and unreturnable serves) are especially important in men's tennis, and are becoming ever more important in women's tennis, too. Tim Henman would arguably have won the Wimbledon men's singles title if he'd had the ability to serve more aces.
The keys to doing so are accuracy and placement choice, more than sheer power. But remember that what you're trying to achieve with an ace is to make sure your ball gets to its destination before your opponent does. So even if you are very good at serving to different areas of the court, if it lacks pace then your opponent is going to return it easily, whether they managed to read it or not.
And whatever your style of service action (Roddick has a very abbreviated action, whereas Sampras had a very long one), speed comes from getting the racket to travel faster as it makes impact with the ball.
Loosen up
If you want a fast serve, the most important thing is to stay very loose, so you can crack your racket past the ball like a whip. But most players are so hell-bent on trying to get the ball into that tiny little box that everything locks up, giving them the opposite of what they need. Or they have pace in their body, but they spread it out through their service action, so there's no real acceleration as they come in to the ball.
Instead, the beginning of the swing should be slow, gradually building up pace, keeping your body loose, and then accelerating up to a very fast and short crack of speed at the top of the contact zone above your head. You should have a square racket face on contact, which will give even more racket head acceleration. From here the follow-through is just deceleration. You should notice how loose the hand and the wrist are, creating that whip effect.
Use your whole body
For some people, standing still, throwing the ball up in the air and working on the coordination of their upper arm, lower arm and wrist can be enough to serve at 120mph. If you've got a decent standing serve but the ball's not travelling quick enough, however, you might also try incorporating a jump, provided you can coordinate it. This means you are pushing off the ground to get more energy going from the very beginning of the action, and then transferring that energy from one body part to another.
This is based on something called ground reaction force, which means that if you push against the earth, the earth responds and you come up off the ground. That then initiates a chain of events up your body, through the legs, the trunk, the upper body and through the upper arm and lower arm, finishing with the wrist, transferring energy onwards and getting quicker each time. But it must be well coordinated. Many people put in a lot of effort, jumping 12 inches off the ground, and still end up hitting the ball tamely.
You could also try tossing the ball slightly further in front of you, to give your body an extra surge forwards. But to maximise the power of your serve, you don't need to use much spin, as long as you do plenty of work on your pace and coordination.
Strengthen your core
A lot of players get little tears in their stomach muscles because of all the energy being transferred from their lower body to upper body. So anyone that is serious about tennis should do core exercises for the stomach and lower back. That will help prevent injuries and improve the transition of energy from the lower half to the upper half, because if you're not very strong in the middle section, when the energy comes through there it can't handle it, and it won't pass on as much as it should.
Now try this ...
If you want pace, you have to unleash it. To do this, pick a sign or something high on the fence at the other end of the court you're playing on, and try to hit it as hard as you can for 20 serves. Forget the service box and work on staying loose, going for absolute power and seeing how quickly you can get that arm going.
After hitting 20 into the back fence, you should be able to readjust your brain, look at serving into the box and say, "hey, let's just go for it." Smack it at that same pace, and see if you can get anywhere near the service box. After five or six serves, if you're within a metre, keep your arm speed exactly the same, but try to add a little bit of topspin, or a bit of slice, to control what you've already got.
Another good exercise for coordination is to squat down, then leap up as you throw a tennis ball. This will help you practise the chain of events that needs to occur, and get a feel for the transfer of energy through your body. You should feel like a spring, letting your legs explode your body upwards.
What am I doing wrong?
Tennis players want a fast serve like people want big biceps in the gym. And you do need to be strong to do it, but it's not about having big arm muscles. Flexibility is what is really important; you want to be loose and have a great range of motion. So something such as bounce press-ups would be fantastic preparation, much better than body-building.