Avoid long minimum periods in the agreement: Don't sign
a contract that has a long minimum period. Anything over eight weeks is
far too long.
You must be able to hold the Estate Agent to account. If they don't do anything in that time you must be able to get rid of them.
Look at the notice period here aswell as often you can't give notice until the minimum period is over. So if you had a 6 week minimum period and a two week notice period you would be stuck with the agent for 8 weeks
Avoid long notice periods. Anything over two weeks is
Only use an agent estate agent who's a member of a trade body like the National Association of Estate Agents, the Ombudsman for Estate Agents, or the Royal Institution of Chartered Surveyors. These have complaints procedures which you can use. Bearing in mind that anyone can set themselves up as an Estate Agent, truly dodgy agents are unlikely to belong to real trade associations like these.
Keep a good written record of all your dealings with your agent. Note phone calls - dates and who you spoke to.