I put a bright pink sticker on my handlebars as I reminder to wear my helmet. I leave my bike on the porch and simply used to hang my helmet on the bike, until I found one of these inside it one day:
BLEGGH. I brought my helmet inside to avoid the grossness but have found that I ocassionaly forget to bring it. Another option I had considered was putting the helmet in an obviously visible place like right by the door but I wanted to get as close to the original, direct, and foolproof helmet-on-bike relationship as possible. This seems to be working for now.
This is just my example of a reminder, it could be modified to your own uses. Remember that the main idea is to create an object/object or object/action pairing relationship to closely resemble the original intended situation. Th example above of putting my helmet by the door didn’t work because I broke the relationship of using the bike itself as a situational reminder. The door has nothing to do with riding my bicycle, and I found that I would just forget to hang my helmet up there.