Hi Lisa- I'm involved with RE agents everyday, but I also have owned a few homes and have used different realtors myself. This is a good question, with many different answers....
$$/ commissions/ rebates- this seems to be coming a bigger issue with the sour economy and low house prices. A lot of people think that if the seller is taking a beating, and maybe its harder for the buyer to qualify, maybe the seller or buyer should go with a low-cost/ discount realtor?
Many buyers might just use the listing agent as a dual agent, or use the listing agent's buddy at their own brokerage/ office.
Referrals are key; they are a big way to get that first meeting/ face to face that you need to seal the deal.
I think having a neighbor or good friend, or a close relative always has an inside scoop on getting the client.
Hope that is helpful. If you'd like to chat offline a bit, feel free.