Read Allan Weiss - he has sales processes for consultants - one of which is getting speaking engagements.
Specialize - specialize - specialize. Be really clear on what people get out of your speaking that is special to you (they can't get it anywhere else). Join associations and give a free 20-minute teaser and then at the end that you go into more detail at your speaking sessions (which you can rent a location and charge admission or solicit companies to have you come in).

Good luck!