A good idea from the inventive folks over at Ju-Ju-Blog: use a tag with a checklist that you keep right on your diaper bag. You can change them up for different outings, or different bags.
Another idea someone had in the comments section was to write down outfits when traveling with children so that you know exactly what's in the bag. Chances are it would be hard to forget something if you have this attached right to your bag. We like the idea of using these as a gift tag at a shower that is part of the gift at the same time.
Ju-Ju-Be is selling the colorful tags they've named BeTagged here for eight bucks, but I suppose if you already have a luggage tag laying around, that would do.