I've had this problem with working out and eating in general. I'll sign up for classes and be super in to for about three weeks and then stop. I just joined myfitnesspal and belonging to this group really helps a lot. I feel that I have to be accountable. When I was working with a trainer, he taught me something that I use in my everyday life. It helps me at work and helps me around the house. He told me that I don't have to think about it or get myself ready to workout. If I tell myself I'll do it in 10 minutes, it will be an hour later. Don't give yourself permission to get ready. Just put on your clothes and do it! This has helped me get over my laziness in general.

Another thing that might be helpful is changing the time of day you do it. I'm not sure if you can do it alone, or what time you are doing it. I found the earlier I do it, the more time I have to move onto something I actually want to do. That means on the weekend I get up, eat, take an hour to digest and do the workout before I do anything else. On the weeknights, I have the DVD in the PlayStation ready to go. I put my clothes on as soon as I get in the house. I don't give myself permission to make dinner or unload the dishwasher until it's done.

Lastly, and this is over the top cheesy and something you might not want to do around your friends (and something my husband endlessly makes fun of me for). I left a motavating message for myself on my cell phone one night after I ran a bunch of miles on the treadmill and felt great. I listen to it when I'm not feeling like working out and try to remember how I will feel once I'm finished.

Also, did you take before pictures? Looking at them might motivate you to keep going.