Not that I've taken my trip yet, but I highly recommend Katie Valk at Maya Travel (you can link to them through the home page of this site - under tours, I think).

She's very knowledgeable about the country, and has aleviated most of my concerns about security, malaria, etc. And, all my questions were answered patiently and immediately. As an independent traveler who's had bad experiences with agents and tour operators, I am thrilled that I found Katie!