Professional speakers know that selling 50 copies to one buyer can be easier than selling 50 buyers one copy each. Try including your book as a part of the registration package or as a premium for the first group of attendees. Be aware that organizers expect a bulk rate for multiple copies. If you are speaking to a local group, ask organizers if the registration fee can be increased a few dollars to cover the cost. Partner up with your hosts.
Most associations have foundations or scholarship funds.
Donate a portion of the proceeds to this group and watch an army of passionate supporters generate sales. Some will buy the book out of support for the foundation. And you will be honored as a hero, generating even more good will and excitement for your program. Many associations depend on vendors to contribute to the event by sponsoring specific items in exchange for additional exposure. Speakers are routinely sponsored, and it wouldn't take much more for those vendors to buy books as well.