My son turned 12 back in February, and so I can relate to the situation a little. He is also a control pitcher. Our coach calls in the piches and locations to the catcher so, it is interesting to watch the success/opportunities for improvement scenario.
TRHit's list is basicly the one we are working on for 2008. My recommendation is to find someone who really knows about grips and finger-pressure and spend some serious time experimenting. This spring we were at Bucky Dent camp in FL again and there is a senior guy there that really knows how to get kids working on this kind of stuff at a young age. He really encouraged them to do this now because there is lots of time to develop some feel and dominance with these pitches. Perhaps one of the more significant things he gave my son was to try a 2-seam change-up which for some (my son was one) will have some significant movement. Note this is not a replacement pitch - it's an additional pitch. As well, there was very specific discussion about where the fingers and thumb (guys who throw a lot should have a calous on the thumb in his opinion) should be placed on the laces for different pitches. Then he got them experimenting with differing finger pressures.
GBMan also did a great job of differentiating between the dominance of change-of-pace over any knuckle effect. It's kind of like the kiddie curve-ball that some kids throw. Sometimes a kid will have some luck with a knuckle or a curve and they will become focussed and forget about their bread and butter pitches. My son is all "chuffed up" about scoops at 1B right now. He made a great one yesterday but, the day prior in a scrmmage, he had a routine hopper hit pretty much right to him; he centered up on it and for some reason tried to reach out and scoop it at the hop - it went right on by...
Back to pitching though. If you were to work seriously on grips (4,2,C/U), finger pressures and experimentation, I would hazard you could be busy all summer. I almost forgot... sitting on the couch watcing TV, flipping a ball in the air and without looking coming to your grips quickly - telegraph prevention.
Cheers,
D