My mother, a piano teacher, always had this on her studio wall.  it is great advice.

Slow Practice = Fast Progress
Fast Practice = Slow Progress
No Practice = No Progress

It is not always about how long one practices but how you practice.  
There is no substitute for repetition, however, most will find it helpful to vary the way in which they practice.  
Example:  Let's say you are working on gaining speed in playing a scale:

1) Get out your metronome if it is not out already.  Select a tempo (speed) that is very easy for you to play the scale.
2) Play the scale with your metronome until you can play the scale 3 times in a row PERFECTLY.  No finger bobbles, no hesitations.  The trick is to do this Perfectly 3 times in a row.  When you have done this move to step 3.
3)  Increase your metronome speed no more than 2 speeds faster.Play the scale until you can play it 3 times is row PERFECTLY
4)  Continue in this manner until you have reached your goal.
Note: there are many ways to practice.  For additional practice tips, check back occasionally or email us.