I don't know about you but if I take the time to think about going to the gym, then I may end up talking myself out of it. What happens is that I start listening to myself say, "Well, you've had a hard day at work, you should take the day off," or "I have to get ready for my trip next week so I should really skip it." Before I know it, I've talked myself all the way home, gym bag in hand with contents totally unused.
I've learned to ignore my inner chatter, which usually consists of a good bit of whining when it comes to exercise, and just go for it and head to the gym. I know this is way easier said than done, but chances are you've talked yourself out of a session at the gym when you really wanted to go. What's more? When all is said and done, you should have more energy to get ready for that trip next week or figure out how you are going to handle that problem at work.
Plus once I've successfully exercised, I also get to ignore me trying to talk myself out of eating dessert after dinner . . .