Do something physical that's not "exercise". Trick your mind. Rather than going for a session of the stationary bike, get your real bike out of the shed and go for a 15 minute ride to the shops. It's just as physical, but it's not "exercise", so that mental hurdle is easier to leap.
Personally, I can't do "exercise". I go a trial membership at a gym and didn't go, I decided to try jogging and didn't, etc etc. But I love my dance classes, and I found out recently I love bike riding. I feel like there's more of a purpose than just "exercise", so it's a lot easier to get up and do it.