How do you keep yourself motivated during a workout?

I try to do my workouts but find myself losing interest half way through and then sort of putting in a half effort. I know this won't help but I can't seem to get over it. When I really try to workout I can do a lot but I find it harder and harder to do this. Please help!

edit: Thanks for all the suggestions, I'm going to try out a lot of these and hopefully be able to stay motivated and focused.