Before I was truly ready to commit to a weight loss program my dad offered me a $3k reward if I stuck to the program and lost my weight so I could buy clothes. I didn't do it but then he said, when I'm ready the offer still stands.
Other ideas might be a trip to the spa for any treatment.
How about a mini trip out of town 3-4 hours for some tourist attraction (this is better for bigger increments of loss)
A trip to your favorite hobby. Ex. Every 5-10lbs go camping, host a game night, scrapbook night, etc.
Every 10lbs a trip to the movies, bowling, girls night out, go kart riding, ice skating, etc
Add to your collection whatever they may be: every 10lbs buy a movie, music CD, stuff for your scrapbooking, new card game, excercise stuff, subscribe to a new magazine, a new book (I rarely buy new books, I just accept whatever is at a garage sale so it is a real treat when I do)whatever...
Put money in a jar for every pound lost and save up for REAL trip like a cruise.
If you want a challenge:
For Every 5 or 10 lbs buy something that cost the same amt as the amt you lost. Example: 20lbs=$20 You MUST buy something you normally wouldn't.
Another challenge: Get a suitcase and every 5lbs go to dollar store and buy 5 things, 10 things, 15 things, etc. It can be some for you and some for others. Put it in a suitcase. Don't touch it until you've reached goal. It can be simple things like a coloring book for your child, a new hairclip for you, a gadget you wanted, a book, some candy (not often), etc. Make sure MOST of it is for you. Really you can do it with any of the above ideas that are things.