When I am shooting with my digital camera I always have a fresh set of batteries in my pocket and a spare Smart Media Card in a pocket carrying case is there also. There are always several spare fully charged sets of batteries in the car or nearby.

Digital cameras eat batteries so you can't have too many and you need to use them up before you recharge them to get maximum life out of them. I also carry a charger (along with my AC chargers) in my car that works off of the cigarette lighter, an hours charge that way can buy you a few pictures you might not get otherwise.