I can tell you first hand, cause I had huge problems trying to generate power on my FH. Is there any particular shot for you where you have the problems?
What I would keep in mind:
1. Don't try to change your technique. Sometimes we try to bring a shot into a different shape mechanically and we think it will help it, but it really won't. Sometimes it can even cost us injuries. I think it is more important for us to just understand easy physical principles.
2. Focus on your legs. If your hand movement in tennis has already become a natural thing for you (so, if you're not a total rookie) try to focus on your legs and torso:
- Bend your knees when playing a baseline shot or serve. Our thighs and our butt are two of the biggest muscles in our bodies and they can generate so much
power. It translates into the shot.
- Use the angular momentum to your advantage. So, turn your upper part of the body (and at the same time bend your knees. I think Dementieva is such a good example, it just seems she overreacts. And you should too, until you settle for a medium, that becomes natural)
- Move into the ball. When you see the ball coming towards you, you already make the move and meet the ball instead of letting it come to you. The ball has so much energy at a certain point, that you won't even need to apply much of your own. Radwanska takes great use of that, or Tomic on the mens side. When you hit the ball when it's already on it's way down, you have to put 100% power into it, and even then you might not get the right depth on the shot.
- Hit the ball in front of you, time it correctly. I don't know about you, but I have great problems with that on my FH. A good exercise for that is if your tennis partner would be ready to toss you balls from behind. That way, you will improve your anticipation and your timing.
- Have your arm flexed fully in point of contact.
- Do a splitstep everytime. Bounce off the ground energetically, that way you will get a lot of energy from the court and it will build up to moments later when you play your shot. The energy basically builds up from the preparation part all the way to contact point. Your shot power builds up from your knees, to your hips, then to your trunk, arm, elbow and last but not least, the wrist. If any part is missing in that chain, your power is not optimal.
- Use the potential energy of the racquet falling. The first thing my coach noticed with my forehand was that I didn't put the racquet up. It should be pointed to the sky. That way, when it falls down and then goes forward to hit, it gets a lot of energy and you can use all the weight of the racquet.