when I shoot a wedding there is always a charged spare and a fresh mem card in my pants pocket....then when I get a second I rotate out with the stuff in my bag....there is a place for used mem cards, a place of dead batteries and then fresh stuff......
Seriously, when I need to carry extra batteries for the SB-800 and the camera, I put them in the softcase that came with the SB-800 and loop that through my belt. Two sets of flash batteries go in there as well as the spare (3rd) D200 battery. Spent or used batteries are just dropped into the bottom of the holder and new ones are in small pouches that fit 4 batteries perfectly or 5 batteries a bit snugger.
You could get one of those paunch-pouches to carry batteries and a micro-fibre towel in that nicely.
That is a good idea bout useing the flash case I may be working with a wedding photographer and when I was shaddowing him I noticed he just threw them in his pocket but that seemed to be a bit of a hassel.
I carry a small bag that fits in my big bag. The small bag contains 2 extra sets of batteries for the flash, an extra camera battery, cards, filter holder, amber, star, and diffuser filters, note pad, pen, and business cards. The bag is about 10 inches by 5 inches. And, I wear the strap on my left shoulder, but the bag hangs on my right hip. It never gets in the way.
Of course, I couldnt fit anything else into that bag if my life depended on it.
Tamrac's messenger-style bag. I feel like it's a holster, and I've never had issues walking through crowded areas with it. You can also add or subtract additional storage pockets as needed, and the interior size is large enough for my 70-300 to be on my 40D and still close properly. Ample size pockets for extra memory and batteries.
I use a small three compartment camera bag I got at walmart for $4 it holds my 6 card case, 8 AAs, 2 camera batt, a lens cloth and some bis cards. it will hook right on my belt and works great for so much. and when I get back to my back pack it just slides right into a pocket just a great solution for me.