First you should know that the speaker bureau business is changing significantly and they can be a good resource for you, but costly when you give up fees. So they should be one way to expand your keynoting business.
National Speakers Association (Mike referred to previously, nsaspeaker.org) has many bureaus coming to check out keynoters and breakout presenters. You'll learn tons of things of high value, beyond your skills. For example, many speakers and trainers leave money on the table when just keynoting. And I'm not referring to selling product, in back. I'm talking about how to present so that the audience asks you for help, consulting, coaching, etc.
Get videos of your presentations, even if the event isn't recording, find someone to video you. If you can convince the meeting planner to videotape you, you'll get a nice pro video for free. I offer to let some organizations use my video (I suggest just clips/ highlights) as samples of their event offerings.
If you have to pay for this on your own, this might be an odd resource, but I've found that many churches (the bigger the better), local to an event, have media professionals who volunteer and can be an inexpensive resource for getting high-quality videos done - regardless of where you're presenting.
I'll mirror Izzy's offer, as well. Give a shout and we can talk.
Find mentors, coaches and associations, and model excellence. I've even paid for professional coaching for keynoting. Victoria Labalme of Rock the Room is a masterful actor's coach and completely transformed how I tell stories. http://www.victorialabalme.com/
Good luck on your hunt.