I have an M5 I bought a few months back, and an E30 that's been untouched for almost a year while I kept our DD's up to snuff and had a fling with a C4 Corvette.
Other than clean the M5 (which was THE most disgusting car I've had the displeasure of cleaning: 2 Dysons FUL of dog hair) I hadn't touched or looked at it until 2 weeks ago. It was a simple, 1hr job of changing steering wheels and fixing the gauge cluster. Suddenly, I WANT to work on it again. I think the key is: don't let the project go cold AND don't bite off more than you can chew. If your heart isn't in it, pick something simple to gain momentum.
The other side is: do it because you want to. We do this for fun, remember?
Finally: In May I bought a TDI golf appliance, and it was probably one of my best automotive decisions. It's cheap to run, hasn't needed ANYTHING, and I'm not inclined to touch it as long as I stay away from TDIforums. Then, when I work on a car, it's much less likely to be at midnight because I need to use it to get to work the next day...

