Don't take a chance of hitting your thumb or finger when hammering a small brad, tack or nail. Slip the fastener between the teeth of a pocket comb first. The comb then holds the fastener while you hold the comb, keeping your fingers out of harm's way when you bring that hammer down. A bobby pin, paper clip or pair of tweezers can be used in place of the comb as well.
When attempting to hammer a nail in a tight or hard-to-reach area, try using a bit of modeling clay (or, in a pinch, a wad of chewed gum) to help hold the nail in place for the first couple of hammer blows.