The easiest, fastest and most stress free way to get to Belize is a direct flight from one of several US gateway cities, the majority of which take about two hours. 
Direct Flights: Flights to Belize are available from Atlanta, Charlotte, Dallas, Houston, and Miami. The major airlines servicing Belize are American Airlines, Continental Airlines, Delta Airlines and US Airways. In addition, direct flights are available from Los Angeles and other Central American countries using Grupo Taca. There are currently no European airlines flying direct to Belize.
Fly/Bus Option: Direct flights to Belize are not however cheap and so if you have a little extra time you may want to consider flying to Cancun, with scheduled and charter flights from the US, Canada and Europe. For example, a recent search on Jet Blue has return flights from New York (JFK) to Cancun for $382US return incl. taxes. On arrival in Cancun you will need to take the express bus to Chetumal. These air-conditioned buses run regularly through the day, have a toilet on board and make a couple of ‘pit-stops’ on the way. The total journey time is about five hours. Depending on your final destination in Belize, you can either cross the border at Chetumal into Corozal, and then continue your journey by road through the mainland of Belize. Or if you are planning to stay on the Cayes, take the express ferry from the centre of Chetumal direct to Ambergris Caye with connections on to Caye Caulker.
By Road : Should you choose to drive to Belize there is much to see and do on the way and there are various routes from the US, across Mexico which vary between 1350 -2000 miles. Regular bus services operate between Tikal and Flores in Guatemala, and Chetumal, Mexico to Belize City.
By Ferry : For those traveling around Central America getting to and from Belize by boat is an option. There are regularly scheduled water taxis that shuttle from Dangriga and Punta Gorda in the south to Guatemala and Honduras.
Wherever your point of arrival in Belize, you are probably going to
need local transportation to your final choice of destination.
Domestic Flights: The fastest and easiest way to get around Belize – Tropic Air and Maya Island Air offer flights all around Belize and this is a great way to travel to your destination with an amazing aerial view of the country. Javier’s Flying Service and Astrum Helicopter also offer transfer service, sometimes directly to your hotel/jungle lodge if there is a landing area.
Bus Transfers: Certainly the cheapest method, the bus service in Belize covers most major towns and highways, with Belize City and Belmopan acting as the major hubs. You will probably need additional transport to your hotel as most are a distance from the highway, but this is often complimentary from the larger hotels and lodges. Most Belizeans use the bus to travel on public holidays when there is also often a reduced service, so check schedules at these times.
Car Rental: My personal preference when traveling on the mainland as the scenery is stunning and you can take the journey at your own pace, enjoying the view and stopping for local attractions. Several of the major car rental companies have offices in Belize, either at the international airport and/or Belize City. Smaller local companies rent vehicles in other areas. We always rent with Budget Car Rental who have a good fleet of well maintained vehicles that are suited to Belize roads!, with many having 4WD. You will require a valid driving license to rent a car and driving is on the right side of the road, with gas stations being available in all major towns. Many roads are unlit, and signage off the main highway is sparse so be careful when driving at night and try to arrive at unknown locations in daylight if possible. 
Boats and Ferries: There are two ferry companies that offer service between Belize City and Ambergris Caye/Caye Caulker. Both operate from the centre of the city and your choice is probably dependent on the time you wish to travel, but boats are available roughly once an hour from 7am to 5.30pm. A ferry service also operates from Corozal to Ambergris Caye (Thunderbolt) once a day. A ferry and private water taxis serve the coastal area of Ambergris Caye between the centre of San Pedro and the resorts lying to the north and south of the island. If you are staying on other cayes (Caye Chapel, Southwater Caye, Cayo Espanto, etc.) your hotel will arrange private transfer.