When I have been successful with weight loss / fitness goals the biggest thing that helped me was a list of rewards. For every 5bls lost, I got a reward. Lots of them were little, but they were things I wouldn't normally do, or get for myself (a Thai massage, new Vejas, whatever) and then some of them were bigger (at goal was a big trip and new wardrobe). I typed it all out, printed it on really nice paper, and put a sticker next to each reward as I redeemed it. This works really well for weight loss, like I said, I did a reward for every 5 lbs. But you can do it for working out as well -- set a goal for the week, and when you have done all of it you get a reward from the reward list.
The last time I posted this, some nasty person came on here calling me a child because of my stickers, lol. But it worked for me.