Door knock and ask for an appointment to preview the house. Be upfront and honest, and state while you don't have an interested buyer now, you would like to make some notes just in case. Say it and mean it. Once in (you may even be allowed in on the spot), make helpful suggestions, and don't talk down to them quoting a bunch of distorted NAR statistics. One suggestion might be telling them about my blog (link below). I know of one realtor who got a listing after telling a FSBO about it. After they read it, they realized it was more work to do it right than they really wanted to deal with.

http://www.trulia.com/blog/rockinblu/2008/08/thinking_about_…