The Office of Fair Trading have some excellent advice in this respect and it is presented in plain English on their Consumer Direct website. Some key tips:
You should shop around for an agent by considering their presence in the market, Sold boards, website, newspaper advertising, etc. Ask for recommendations from friends and relatives who have recently bought or sold property. Most of all choose an agent who sells your type of property and offers a contract that suits you.
You could use an estate agent who is a member of The Property Ombudsman (TPO) Scheme. You can usually find a least one TPO member in the high street. Scheme members follow a voluntary code of practice which has been approved by the OFT and means that, if things go wrong, the TPO will help sort out the problem.
Having made your decision you will be asked to sign a contract. However, before signing, read the contract carefully and make sure you understand it. Find out whether you have the right to cancel the contract. Check how long it will run. The contract should run for a reasonable length of time that is sufficient for the purposes of marketing your property effectively and finding potential buyers. Beware of contracts that tie you to an estate agent for a very long time.