I use a few techniques to increase speed on a particular piece:
1) Practice hands alone at first and practice the fast (eighth/sixteenth note) passages in the following rhythms:
The secret is to do ALL the rhythms so that you don't end up playing passage in one particular (wrong) rhythm.
2) The metronome trick. Play either one hand or both hands. Start at a slow tempo with the metronome and increase two clicks at a time until the tempo desired is reached.
3) Play slowly. I know--paradox...but, if you can play the passage at a slow tempo perfectly, you know that it is in your head, not just in your fingers
If using this "trick", it's probably best to use the metronome trick to get the tempo up to speed.
These are just options...have helped me in the past, but I can't guarantee they'll work for everyone or in every situation. I know that all three have been taught and used for many many years...if it's not broke, don't fix it
[This message has been edited by Josh Allen (edited 16 July 2000).]