There are some things that have always helped me.....but mostly, it's a drive to be in the best shape possible, all the time, for me. If you don't have that internal drive, then maybe some of these things can help?
Set short term goals. I always have a big race coming up, or some kind of public event.....about a month apart, that I focus on. If you give yourself short-term goals, and put them end to end.....pretty soon you realize you have been actually meeting your long term goal.
I have fitness magazines all over my house. I look at them when I am feeling un-motivated.....seeing all those fit models and reading success stories will always get me up and to the gym, even on days where I'm just burnt out.
Seeing real changes happening is a huge motivator. If you haven't seen a lot of changes in your body for the amount of work you are putting in, then perhaps you should review what you are doing, and change things up to be more effective. Once those changes start happening, it just helps to motivate so much more!
I can't/will not work out without my Ipod. I have a ton of music that motivates the hell out of me, and it's a crucial instrument in my daily workout. Music can be HIGHLY motivating, and if you don't already do this, then this is the first thing I'd change.
That's about all I can think of right now....like I said, I just have this internal drive to challenge myself every day, and working out is one of those ways I do it. Not everyone is like that, but hopefully these ideas might help a bit??
Good luck!!