Make sure the agent has experience of selling property like yours – and the best way to do that is to check there are properties similar to yours in the window. There's no point using an agency which sells lands, if you have a block of apartments to sell.
Go into a number of estate agents and pretend to be looking to buy a house similar to yours – are they professional? Do they seem competent? Are you impressed?
Look at the agent's website – are the pictures well taken, and the descriptions clear and relevant?
A good agent will invest in marketing to ensure they get the best price, while a poor one will just wait for customers to come to them.
Do not feel pressured to hire the estate agent you bought your house from. Obviously, if you were impressed how they sold your house to you, you might decide to go with them again.
What is their viewing policy – check they will accompany potential buyers when you are out. Do they phone and send around potential buyers straight away?