I stay motivated by one, keeping a picture of my goal weight virtual model on my fridge. You can make one too at www.virtualmodel.com, it's free.
Two, I have a pair of jeans 2 sizes smaller that I bought and want to get into by the summer and last but not least, the fact that I am tired of being over weight is enough to keep me motivated.

I want this real bad and will exercise my butt off till I reach goal. I exercise 5 days a week and I assure you, 2 of those days I may be in a bad mood for what ever reason, didn't sleep well, kids are nagging, the point is, I do my exercise's anyways and by the time I'm done, I'm in a better mood.

Plus good music really helps too.