This site has some great tips on how to get better at drumming on Rock Band.  Obviously the drums are the hardest to play because it is the most realistic instrument in the game.  It is hard to develop a independency of your arms and your foot but this has a few tips and as it suggests it is not something that will happen over night.  It is all good until you have to go up to the next level in the game!  I have new respect for drummers!