This article has been around for awhile but the information is still valid: Working With An Agent.

The point is this: regardless of how wonderful a talent your are, the commissions you pay a booking agent must be able to not only cover the agent's expenses related to their representing you, but must provide some degree of profit for them. If you as an artist cannot provide an agent a profit, then you are asking the agent to make a financial investment in you and your career. Think about that for a moment.

I think some artists believe that an agent can get them better gigs, and in doing so increase their audience. While the first part is probably true in most cases, the hard truth is if you're not drawing a crowd now, you probably aren't ready for those "better" gigs anyway.

In most cases it's better to wait, and focus on building your local audience as much as you can. You really don't want an agent coming to check out your band and finding an empty room.