My advice would be to find something you enjoy, whether that is doing aerobics tapes at home, joining a spin class, or just walking around your neighborhood.
Second, make time in your schedule for it. For me, this means I do it first thing after I wake up, while my brain is still too sleep-foggy to argue with me about it. By the time it figures out I am actually doing exercise, I am half-way done.
If you find yourself unable to motivate yourself, maybe having a work-out buddy would help you. This could be a friend or neighbor who is willing to meet up with you for work-outs. You'll be less likely to flake out if you know someone is waiting for you to show up.
Along the same lines, maybe getting a few lessons from a trainer would help you.
As for me--I'm a loner. So, I try to do things I think are fun and/or different every day. I don't do the same workouts every day, so I don't get into a rut and get bored.