There's always the
Alpo Diet 
First and foremost, myself, of course. Beyond that, I'm generally a sucker for peer pressure, so once I hooked up with a group of runners (who I found through dancing) and heard them talk about all the races they ran and good times they had, I started trying to keep up (and have started to surpass, in some cases). I also make a race calendar as far out as I can and publish it on my MySpace profile, and I always write a post-mortem blog on each race including finishing times from previous races as kind of a running log (no pun intended) on my speed progress.
I guess it doesn't hurt to have a girlfriend who also runs, is competitive as hell, and smokes me in 5Ks, which is fine because the Charleston 10K is coming up, and she hasn't competed in anything beyond 5Ks yet