The BEST article I've ever read on practicing is the one by John Petrucci in his Wild Stringdom book.
Absolutely fantastic. How he does it, is he accumulates a large filing cabinet and organizes all his material in 3 folders. Technique, Theory, Music or something similar.
Within each folder are smaller folders...for example, technique looks like this
Technique --->
-Alt. Picking
-Sweep Picking
-Legato
-etc...
And within each mini folder he has various exercises that he found interesting and filed away.
When it comes time to practice, he picks a topic, picks a random article and works on it for 15 mins or more, and then moves on. His exercises are different everyday, which keeps him interested, and on edge.
I've done this, and I now have a filing cabinet organized in a similar fashion, and it really does work wonders
. Keeps you from playing the same exercise over and over.