Ask friends who they recommend. Interview more than one. Ask about their sales record. Do the pre-research on your own, and set realistic expectations. Use the web to see what's out there and go to lots of open houses. If you are selling, spruce up your house, get rid of knick nacks and personal stuff like pictures, religious items, etc.
We are on house #5. Three out of the four we sold were all sold in horrible markets, within 3 months because we had good agents and we didn't expect to become millionaires from the sale.