First and foremost, you should feel comfortable with the person you choose. They should be able to demonstrate their knowledge and value. This includes doing the necessary research to help you price your home and come up with an effective strategy to sell your home. These same skills can transfer to helping you make a good investment as a buyer.
i love my current realtor. he is dependable, knowledgable, professional, candid, detail oriented, super organized, and returns all communications promptly. small things like having a pen and a bottle of water ready when we go out for a string of showings (esp in 90 degree weather) was really great.
When searching for a Realtor I believe it is important to interview a few and get a feel for what there knowledge of the market is and what tools they have to keep you informed.. also ask for references , you want someone that is readily available, and organized.