Be consistent. Practice a little EVERY day. This is far more beneficial than one or two long practice sessions a week.
Schedule/Routine. Have a certain time each day that is set aside specifically for your practice. It will then become part of your daily routine. Split your daily practice into even smaller time chunks, i.e. technical work in the morning and longer pieces in the afternoon/evening.
Small Sections. Learn each piece a phrase at a time. Practice each phrase S-L-O-W-L-Y until you have it, and then go to the next phrase.
Quality vs. Quantity. Starting at the beginning of the piece and playing through to the end each time you practice is not an effective use of your time. You are merely practicing mistakes! If you keep practicing your mistakes you will be very good at playing them.
Mix it up! Sometimes start in the middle of your piece and work to the end, or start with the last phrase and work backwards by section.
Record. Regularly record yourself playing and listen carefully to it.
Slippery Spots. Each piece has a tricky or technical section. Focus on the these phrases rather than just the sections that you like to play.
Listen. Find recordings of your piece being performed by excellent players.