We all have things we like to take on every photo shoot. If I am going further than my own yard - I like to take the following items in addition to my camera and lenses.
1) Rocket Blower - This duster is shaped like a rocket, and happily sits on its fins when not in use. It also fits nicely into a camera bag. I use this to blow off the mirrors and sensors inside the camera. Also, a quick blow on the outside of a lens will remove most dust without using a cloth. Not good for fingerprints. (
Giottos)
2) Heat pad - I use a small heating pad, those filled with magic beads, as a steady rest/prop for my camera when doing macro photography. Much like the 9"x12" model shown here:
Thermalon.
The pads conform to rough terrain and make a stable, clean rest for the camera. You can also use them for sore muscles.
3) Off camera flash - I have a Metz 48 flash which I use with a Manfrotto flash bracket. I use the flash for fill quite a bit. I have a homemade diffuser made from a plastic tub and some wax paper. It softens the light nicely.
4) Extra batteries for camera and flash. We all know the benefit of extra batteries.
5) Extra memory cards.