Had mine listed on the VAC classifieds, AirstreamGuy.com, Craigslist, a video on YouTube and the classified section here at the forums.
My buyer happened to find me through these classifieds, but all five sites I mentioned are free so I say start postin'!
Advice for your listing? Off the top of my head: the more big, clear photos you can include the better. And I would imagine another great way to stand out from the crowd might be to offer a PDF of a "CarFax" type independent report, which you'd have to have written up by a reputable RV dealer. Could give people good peace of mind to hear that an RV mechanic has verified that all your systems work.