1) Keep your soaps in a dry, cool place. This will allow your soap to continue to curing. The longer a bar cures, the longer the bar will last.
2) Use your soaps with a washcloth or soap saver. This will help keep your soap out of the water stream.
3) Use a soap dish. This will allow your soap to dry off in-between uses vs. staying mushy in the shower.
Overall, the key to a long lasting soap bar is to keep the soap dry!