When I'm in a 'don't wanna" mood, I tell myself I'll only do half as much time as normal. Or I won't go as "full steam" as usual. 9 times out of 10, once I get moving it feels good and I get a normal workout in.
I do the same thing as Gina - this morning I said "I'll only do 30 minutes and keep it at a low heart rate" Three minutes in I changed my program and hit the intervals... And I feel better for it!
Carinna, you really don't want to spend the rest of today with the thought that you didn't work out hanging over your head, do you? I don't know what your usual routine is, but I'm pretty sure it can be over and done within an hour. Then you have the rest of the day to feel great about yourself. Sure beats spending the next 8 hours or so feeling crummy about what you didn't do!