I was just going to post the info from the BelizeBus site (my site), but someone just beat me to it. I haven't updated the times for a while, so contact the agent to confirm departure times.

I can suggest a slight variation to Uncle Buck's earlier post.
Tropic Air now fly almost to the border (Maya Flats airstrip between San Ignacio and Benque). Continue to the border for a few bucks by bus or taxi, and then on the Guatemalan side, hire a Guatemalan taxi for about US$50 and go directly to Tikal (most of the public transport will go to Flores).