Pool your financial resources. Booking agencies don't work for free, unless they're desperately trying to court you or are more of an indie outfit. Determine among your band members how much money you have and how much you are willing to spend. The cost of hiring a booking agency can be expensive. The old adage "you've got to spend money to make money," applies here, as some booking agencies will cost you money upfront. On the other hand, a solid booking agency will be able to get you higher-paying gigs, of which they will most likely take a certain percentage. You should end up making more money overall than you did before if they're doing their job right.
Do research. Talk to other people and bands in the business. Working with an agency with a solid reputation and/or with one people have worked with before is more promising than hiring someone you've never heard of, no matter how big or important they claim to be. Look at a variety of agencies that fit within your budget and seem suitable for a band at your specific level. If you're an indie band just starting out, don't bother with big-name agencies working with huge artists as most likely you wouldn't be able to just "hire" them; they'd come looking for you.
Put together a press kit. This should include your CD, photo, bios, press clippings and any other material that represents your band. Like your band, booking agencies are putting themselves at risk by working with you. Most reputable booking agencies will only work with clients that can prove that they are a professional band worth working with. An exceptional press kit can help convince booking agencies you are worth working with.
Contact the agencies you feel will work well with your band. Don't just email. Call them. You are putting part of the future of your band in the hands of a booking agency, and you need to talk to someone directly. Arrange a meeting with a booking agent.
Have an in-depth discussion with the booking agency you wish to hire. Present them your press kit and key information about your band, such as prior show history and album sales in particular regions. Let them know your intentions for hiring them, such as where you'd like to tour or which big clubs you'd like to get to play. Ask them every question you can think of.
Hire a lawyer. Have the lawyer look over any contracts a booking agency presents you and help make any necessary changes before signing.