The easiest way to get a serious boost would be to get a competitive one-hander to use in the offhand, or a two-hander - the problem with that being, of course, that Barbarians in Inferno are already in serious trouble with a shield, even with a load of armor and resists.
The biggest damage booster out there, then (aside from stacking tons of extra strength), would be items with attack speed - by far the most powerful stat when you have a high DPS weapon. Gloves, rings, and amulets are the most common (not sure if they're the only) places to find it.
Substantial amounts of +crit and +crit damage are a little harder to find on otherwise good items, but with the Barbarian's mechanics and passives, they may well pay off.