Stay healthy:
Belize is a relatively healthy country. Bottled water is a must in most areas. And, unless you eat only at ultra-touristic restaurants, dysentery will probably strike at some point; be prepared with over-the-counter medication and prescription antibiotics.
The CDC lists all of Belize except Belize City as a malaria risk area, and recommends the antimalarial drug chloroquine. Other drugs may also be recommended in certain circumstances - consult a qualified professional specialist.
Insect/mosquito bites should be prevented with appropriate clothing, repellents and insecticides, and bed nets if sleeping in non-air-con/unscreened rooms.
The sun, as anywhere else in the tropics, is very intense. A hat, high-SPF sunscreen, and sunglasses should do you fine.
Many places in Belize are very hot and humid, and dehydration is a risk. An expat suggests to drink as much water as you want, and then drink that much again.
The adult HIV/AIDS prevalence rate is currently at 2.5% or 1 per 40 adults, which is 4 times higher than the USA and 25 times higher than the UK. Safety First!
Respect:
Belizeans are some of the most socially relaxed people in the world, especially if you venture inland away from the tourist islands of Ambergris Caye and Caye Caulker. The pace of life is generally slower in Belize, so it's good practice to begin any social interaction, even to ask a quick question, with eye contact and a genuinely pleasant greeting. Most rural Belizeans enjoy casual conversation and you could easily find yourself chatting it up for a few hours. Hey, it's part of the charm!
The Maya communities can be a little more reserved at times. As always, a little respect and politeness will carry you through.
Contact:
Payphones are the most common public phones in country, and accept pre-purchased phone cards.
Internet cafes can be found in larger tourist areas, but are infrequent in rural areas. Keep in touch with expats at the Central America Forum.