1) More is not better
Contact a minimum of three and an absolute maximum of six recruiters - any more and you risk losing control of your job search, with different recruiters sending out your CV for the same vacancy. That's something banks really hate.
2) Get specialist
The recruiters you deal with should specialise in your job function and have solid experience. Don't think someone is an expert in, say, risk management, just because the job title says so - he or she may have only recently been assigned to the sector. Choose someone who has been recruiting in risk for a number of years. Look at recruiter biographies on the firm's website, or on LinkedIn.
3) Consider niche firms
Also remember that although it's probably the large, mass-market agencies who will show up on a Google search, niche firms may be just as good. Leading consultants from the big players often end up opening their own boutiques, some of which will focus on your job function. So don't rush your research of the recruiter market - take the time to see who's really out there.
4) Ask them who they work for
It's important to discover from the outset which banks your recruiters represent and which they do not. If someone says "every bank", walk away: it's just not true. They should be able to name a few key employers and answer your questions about them.
5) Test their knowledge
Have a chat about your job function to see if they can talk the talk and understand current issues, both technically and from an employment perspective. Ideally try to find recruiters who have worked in the financial sector themselves.
6) If they're good, stick with them
Just like bankers, recruiters are prone to changing companies. As a general rule, if you have a good relationship with a recruiter, stay with the person, rather than his or her firm. It's the individual consultants who usually have the industry contacts and the knowledge that you need.