A mix of large, well-established generalist agencies with one or two specialising in your field should ensure you are covering the market well.
Word of mouth by other contractors will give you an honest opinion (either colleagues or via the CUK forum).
The Recruitment and Employment Confederation (REC) is the watchdog of the agency marketplace if wish to check the agencies you plan to approach are members.
We also recommend that you:
Speak to the Agency
Do they create the right impression? If they don't impress you they won't impress employers either.
Do they understand what you're looking for?
Do they seem to understand the IT market?
Do they seem keen to work with you?
Are they willing to meet you to discuss your requirements?
Do they guarantee not to send out your CV without permission?
Do they keep you updated?
Get to Know Them
Each agency will likely have many skilled applicants on their books. If you can develop a business relationship with one contact at the agency then get to know that person. Call them regularly to keep your name at the forefront of their mind.
Don't be afraid to ask what their margin is and be prepared to negotiate if you are in a strong position (some agencies are on fixed rates with their clients however).