I do one freebie a year for a Registered Charity, on the proviso that I get an advert in their association literature, and a mention in any press coverage, I find that I get at least 2 or 3 bookings from the advertising that we agree, which more than pays my expenses for attending the free event

, so we all benefit.
For other charity events, I tend to offer a small discount for booking me. Those charities who tend to book me for their non fundraising events, staff parties etc, obviously get a bigger discount due to their loyalty. This also ensures that your roadshow doesn't just get taken advantage of, for the cheap events, and then they go off and book another D.J for the full priced ones

.
One Charity I worked for, was once good enough to put a scaled down copy of my business card, on the back of their event tickets

, in return for offering our services for Free. Since they sold a little over 800 tickets to one Gala, this was very effective advertising and worked well

. Don't be afraid to negotiate free advertising in return for your services at the event.