Keep slogging! Seriously, it ain't easy. Clubs use booking agents for the same reason landlords use real estate agents - to screen out the 'riff-raff'. Put a comprehensive press pack together - including a DVD and CD, photos of your band in action and links to your online videos and music, FB page, etc. and keep sending them to the agents. Most will look at your online link for less than a minute to make a quick judgement on whether you deserve any further attention, so make sure you've got your best stuff ready.
If your music regularly attracts large crowds, you don't need to hunt for an agent because they will hunt you. If your music does not attract crowds, they are not interested. Agents are a lot like banks. The time when they are most interested in doing business is when you don't really need them.
We had a good relationship with a few booking agents over the years and they were effective at keeping the performance fees relatively high and bringing regular bookings. Now days online virtual agencies like Gigmasters and Gig Salad are pretty effective for a monthly fee.