Stay healthy:
Drink only purified water (Agua Pura Ecológica is recommended by most of hospitals and hotels).
CDC states that malaria risk exists in rural areas at altitudes lower than 1,500 metres, with no risk in Antigua or Lake Atitlán. Preventative anti-malarial medication can and should be purchased ahead of visiting malaria-endemic areas.
Dengue fever is endemic throughout Guatemala.
Hepatitis A&B vaccinations are recommended.

Respect:
Address people you don't know in a formal manner (Señor, Señora, Usted), and greet people in the following way:
day - "buenos dias" "feliz dia"
night- "feliz noche" "buenas noches"
You'll encounter this in more suburban, rural areas. Native Guatemalans are raised to greet strangers formally.

Contact:
Phone:
Guatemala's international calling code is 502. There are no area codes. Phone numbers all have eight digits. On September 18, 2004, the phone system switched from seven to eight digits, and there is a scheme for adding specific digits to the front of seven-digit numbers (WTNG.info description).
The phone system isn't great, but it works. Tourists can call abroad from call centers, where you pay by the minute. It is also easy to purchase a calling card to use at public pay phones. The phones there do not accept money, so to use a public phone on the street you must purchase a telephone card. Typically, the cost is around 8 quetzals for a 10 min call to North America. Cell phones are quite cheap and calling to the US through one can get as low as $0.08 a min. If you are planning to stay for a while and plan to use the phone, you should consider buying a cheap prepaid phone. Wireless nation-wide internet access for laptops is also available as a service from some companies. Telefonica has good coverage with their PCMCIA EV-DO cards.

Post:
The post system is traditionally not reliable, but your post cards usually get through. A stamp for Europe is Q5. There are; however, many other alternative companies to the federal mail system that are reliable, though frequently somewhat pricey.

Internet:
Internet access is widely available. Even most of the more remote areas have some type of internet access available. Many larger areas also have WiFi. All of the Camperos chicken/pizza restaurants (which are numerous) offer free WiFi, as well as many other restaurants and cafes. Some hotels may also offer computer banks with internet access. Just ask and you eventually will find some sort of free access.