At first I tried a reward every 5 or 10 lbs, but I ended up discouraged because my weight loss has been very, VERY slow...usually .1 to .3 lbs per week. So it was taking FOREVER for a reward or treat on my little reward schedule, so I had to adjust.
Now I have lists, like many others' lists I read on this thread. I have two, though. One list is very small, inexpensive things I can treat myself to for every 1# lost---like those Bonne Belle nail polishes and lip glosses that are usually less than $1 each, or $1 organic fruit leather (1 pt for two pieces!), or something like that. Small, but makes me feel like I've accomplished something.
Then, for bigger goals (3-10lbs) I have a second list...it's a wish list of bigger things that I've been wanting, and I add to it whenever I think of something. This list has included new shirts, a pair of shoes, specific books/movies that recently came out, good-quality makeup (have I mentioned I love makeup?

) , going to see a movie, jewelry, household stuff (believe it or not, treating myself to decent cleaning equipment has worked for me in the past!! It's amazing what you wish for when you're trying to scrub doggy drool off a windowpane with water and toilet paper!), etc.
For my "final" goal (my current goal is 175, which is a loss of 10% of my starting weight), my dad has been very supportive and he has said that when I reach the 20# mark, he will give me some "fun money."

So when I reach that goal, I'm going to take a photo of the scale reading, and email it to him. My family has been very supportive of me, as most of them are trying to get fit as well, so that has been probably the best reward!