We flew twice this summer back to the US thru Cancun because of the price. Fly or take the Thunderbolt to Corozal. Contact Henry Menzies, Belize VIP Transfer service: firstname.lastname@example.org - 422-2725 - in advance. He'll meet you at the airport, get you thru Customs and Immigration to the bus station in Chetumal. He also knows the bus schedule and can get your ticket. It's about $25US/person. Takes 5.5 to 6 hrs. bus is luxury altho over A/Cd - bring a sweater. Shows movies mostly in spanish, some English with subtitles. had one in Danish with Spanish subtitles. There are food vendors at stops - always Tulum and Playa del Carmen. Sometimes there's a milk run. You can get the schedule at: http://www.ado.com.mx/ado/index.jsp
If your plane goes early in the morning, you might consider spending the night. There's a nice hotel call Hotel Kin Mayab which is across the street behind the bus station - an easy walk. We stayed there and had one room that was $60US and another that was $39 - which was out back and just as nice. There are stores and restaurants all over near the bus station and when we were there, federales everywhere.
On the return, you can call Henry from the phone box store across the street once you know which bus. His guys aren't allowed at the bus station in Chetumal (territorial fight) at the moment but they will meet you at the Mexican border. It should cost 100 pesos to get to the border.
It makes a long day (which is why it might be better to go the night before or, if it's a late plane back, stay the night you get there) but it's very do-able.