I know this gets old but....moderation....you want a sweet have it but don't gorge...then push harder next workout....also don't say diet...I think if your not in contest prep mode it just screws your head up...just think of it as healthy eating.
Or just think "damn I can't have that,my ass won't fit in my dolce and gabbana jeans." it works
The only time I strictly stayed on a diet for a long time (10 weeks) was in preparing for a show. Knowing I would be onstage in a bikini was the biggest factor in helping me to eat clean. I don't have a problem w/getting motivated to workout - I love it. Eating clean and minimizing wine consumption is the problem.
Competing is not my thing, but I'm very glad I did it. It was fun, and really fun to wake up each morning to see how my body had changed.
But as for motivation, one thing that really helps in motivating me to eat clean is reading Oxygen Magazine. I have tons of back issues because I always save the ones Adela is in. If you don't subscribe to Oxygen, you should.
Also, I look at people who eat clean for a certain amount of time and see how great they look and feel, and realize that not eating clean for that same amount of time just ends you up in the same place; feeling like a fat bloated frog. Why not just eat cleaner and feel and look better?
If you're not doing a show, just "clean up" your diet or eating regimen. Instead of ice cream for desert have strawberries w/splenda. Make your own salad dressing w/one tablespoon of oil (flax or olive etc), one T mustard of any flavor, one T sugar-free maple syrup and 2 T balsamic vinegar instead of pouring on the bottled kind. You can alter that recipe any way you like, adding garlic and/or rice vinegar etc...but keep your serving of oil to 1 T. There is a healthy recipe thread on the nutrition board.
Eat 5 or 6 small meals (approx calories from 250-300 cal) and eat 2 1/2 - 3 hours apart. This was a hard thing for me to do since I usually ate 3 huge meals a day. But the smaller, more frequent meals get your metabolism going and keep it revved. Plus, when you are dying for something to eat, it's time to eat again! If you want something sweet and satisfying, have a protein shake for that meal. I like Whey Gourmet the best...I just think it tastes the best.
Another motivation is having a good personal trainer. Make sure you look carefully at your trainer and learn about him/her as there are some that I wouldn't choose.
Have fun and remember, if you eat pretty clean for just this week, you'll look and feel better. Then do it next week, and the next. You are not dieting for a show so you don't need to be extremely strict. Just make better choices.