After the finish, don't let people bother you while you are compiling the results. If necessary, have a helper run interference for you. Announce the preliminary results at the race, and then pack up and go home. Later, you should enter the results in a database and print out results overall and by age, gender, and category. Post these results to your answering machine, the WEB, email, local papers, and to the local skate and bike shops.
People like to see their names in print.
Making the results highly visible will ensure a high turnout for future races. Another idea is to keep a tally of points accumulated over the course of a summer-long series and to periodically post standings. Brainstorm to figure out better ways of executing future races. Use your event log as the basis for these discussions. Talk about new rules, new courses, different race types and anything else you can think of.