If I find I really don't want to go to the gym, I'll just say, "Fine, I won't go. But I'll put my gym shoes on." Once I have my gym shoes on, somehow I'm out the door. If you can't bring yourself to start working out, just try to take one small step in that direction. The first step is the hardest!
I learned this in a group therapy class I did in cognitive behavioural therapy when I was suffering from depression. Sometimes the whole behaviour is too daunting, but you can ALWAYS do the first step.