Record a demo if you don't have one yet. You will need several songs recorded to a CD or uploaded to a website so a promoter can hear your music on demand and not have to wait for a live show. You should also have a current band photo and total song list available to form a mini press kit for the promoter to look at.
Talk to other bands that have good turnouts at shows and ask if they use a promoter. Call up clubs as well since many of them will use promoters for events and they may be able to recommend someone they are happy with.
Contact promoters that you have heard good things about and ask them about promoting your band. Ask about their contacts and how they promote their clients. Give them your demo so they can listen to your music and see if it's something they are willing to get behind.
Sign a short-term agreement to start with as a trial to see if both sides are happy with the arrangement. When the initial agreement ends, you can either sign a longer-term agreement or you can look for a different promoter that will better fit your needs. Some promoters only work on a per-gig basis, which allows you to switch to a new one at any time if you are not satisfied.