This is a link to the .pdf attachment I got with the notice from San Diego off Road Coalition with a sample letter and the e-mail address to send it to.
Short and sweet, my experience is that long letters don't make much difference, in fact, the shorter the letter the more likely it will be read. Form letters are fine. many years ago I was a volunteer in a local politcians office and letters are always checked by staff, a very select few are forwarded to the HMFIC all the rest are tallied up X# Pro/X# Con and that's what really matters. You'd be surprised by how
few letters are usually sent in causes like this. Themore we can get out there the better
http://www.sdorc.org/issues/corral_notice.pdf