try this excersises:with your heel down(planted on the pedal)play continuosly a bar of quarter notes,a bar of 8-notes,a bar of triplets and a bar of sixteens.do this for 15 minutes(or more) a day,at a COMFORTABLE tempo(the use of a metronome is more than suggested).it is important to stay relaxed and not force things(remember, it is not a race. you are trying to build muscles).
After a small break start playing quarter notes(without metronome)again at a comfortable speed.slowly and gradually start accelerating and also increasing volume. go back and forth for a while and then stay at a relatively fast and loud level for as long as you can.Now,some say that when your foot starts burning you should stop.Some say that this is the time to push more because this is where you build stamina and power.The middle solution works for me the best.when your foot starts burnin' back off a little bit(both in speed and volume). this excersises build stamina,speed AND power.
No matter how you play, you should practice these excersises heel down because there are important muscles to be built.you can also try'em heel up but you ARE gonna see an improvement anyway.Good Luck
