My grad school roommate had a wonderful philosophy on practicing when you don't feel like it: "just try a little." 5 minutes. 10 minutes. 15 minutes. If you don't get past that point, at least you tried (and some practicing is better than no practicing). But chances are 5 minutes will turn into 10, to 15, to 20, and maybe more, as getting started is often the hardest part.
Here's a neat blog entry from The Bulletproof Musician that does a great job explaining this.
Elizabeth Borowsky is a pianist, teacher, and composer. She is a Nationally Certified Teacher of Music in Piano (Music Teachers National Association).