I use "the corner rule" for this and other exercise. The rule is that I must get dressed and get out there. But, if I get to the corner and *really, really* don't feel like going further, I'm allowed to go back. Guilt free! The point being, of course, that getting out is the hardest part. It's never failed me...

Also, having a good training group helps. It's hard to ditch when you have a group of wonderful women (or men, but not in my group!) waiting for you and missing you if you don't show!