Going on vacation to the French and Italian Riviera is a strong motivator to lose weight, but I couldn't have done it without my Treo and a little program called "Diet & Exercise Assistant".
I am not saying this would work for everybody, because one has to find out what motivates him/her to lose weight, but for me, knowing how well I was doing every day really helped. Every day I used the program to find out how many calories I "lost" compared to how much my body used, and I knew for sure that I would lose weight. It's just simple math, really; if you eat fewer calories than you burn by just sitting all day (I work in an office), than those calories have to come from your "reserves".
Diet & Exercise Assistant has a food database that you can use, and you can customize it to add the foods you eat on a regular basis. Inputting all this becomes really simple and fast once you have entered your favorite foods in there. And you can make sure you get enough protein, fiber, etc. Very Good!