This is my little trick. I told it to DD and I'll share it again. It helps if you are careful with your budget, like I am! I think in banking terms. As in NO CREDIT!!
Say I want something really good, like those brownie bites from Costco that are 130 calories a pop and you could eat at least four of them. I think to myself if I want one/two/three+, I have to "pay" for them. Otherwise known as I have to exercise 130 calories' worth. OR I CAN'T EAT ONE. No pay, no play. No credit. Which means I can't say to myself, Oh I'll eat a couple and this weekend I'll take a 20 mile bike ride. No. If I eat one today, I exercise today. Cash on the barrel!
I swear that sometimes I am really lazy and I could be bothered less about exercising so I skip the goodie. I don't feel like working it off. Simple as that. Other times it's like the proverbial carrot at the end of the stick. Dangle that carrot (the brownie bite) and I'll follow it (exercise).
See? Keeping in shape is all about thinking in logical terms.