Battery caddy

These battery holders seem like an effective way to carry backup batteries for flashes or what have you. If you're someone who shoots weddings these could save you some precious seconds if you need to do a quick-change.

http://www.personalbatterycaddy.com/html/SlimLine.htm

Quoted from personalbatterycaddy.com (empasis mine):

• Six different versions hold 4-9V, 4-AA, 6-AAA, 4-C, 4-D, or 4-CR123 batteries.
• Store your batteries in a compact, easy to find caddy.
• Originally designed for pilots, but also great for camping, boating, travel or at home.
• Dispenses batteries with one hand for safety and convenience.
• Terminal protection at both ends regardless of how the battery is inserted.
• Made of high quality engineered plastic.