Problem barking is generally defined as barking constantly for a period of half an hour or more, or more specifically, any prolonged period of barking that irritates your neighbors.
Leave the radio on, or the television, to give your dog a sense of company and normalcy.
Give your dog something to occupy him while you are gone. A Buster Cube, or a stuffed Kong works well as a time-consuming, entertaining toy.
A companion. Two dogs will often keep each other company and entertained when alone, although I really don't recommend this option unless you are prepared to deal with all that having two dogs (or more) entails.