You know how there's that thing you don't really like doing, but yet people keep asking you to do it? And it's taking time and energy away from that thing you really love doing. Start charging people enough to make it worth your while and to discourage those folks who don't really want you to do it, they just want someone. Like author Neil Gaiman:
Q. How can I get Neil Gaiman to make an appearance at my school/convention/event?
A. Contact Lisa Bransdorf at the Greater Talent Network. Tell her you want Neil to appear somewhere. Have her tell you how much it costs. Have her say it again in case you misheard it the first time. Tell her you could get Bill Clinton for that money. Have her tell you that you couldn't even get ten minutes of Bill Clinton for that money but it's true, he's not cheap.
On the other hand, I'm really busy, and I ought to be writing, so pricing appearances somewhere between ridiculously high and obscenely high helps to discourage most of the people who want me to come and talk to them. Which I could make a full time profession, if I didn't say 'no' a lot.