My favorite tricks don't have much to do with exposure, at least not directly:
1. I always carry a keychain size flashlight, either a Mini Mag-Lite or one of the nifty newish LED lights. Even if I'm not shooting at night or indoors there are times when extremely contrasty bright sunshine can make it difficult to read the aperture ring, shutter speed dial, etc.
2. I *nearly* always carry a bubble level. Mine's an inexpensive, small but reliable Credo Pocket Level with two bubble vials.
3. When shooting 35mm I note (either directly on the cassette, on a bit of masking tape or a note tucked into the film can with the film) the EI, date and which developer I had in mind for a particular shooting session.