Learn Spanish. Okay, I know it's not always possible to learn a whole language before you take a trip. I'm not fluent in Spanish myself. But knowing some level of Spanish goes a long way in being aware of your surroundings, communicating, and navigating what can be a confusing destination. Check out About.com's Spanish site for some free Spanish lessons.
Walking at night. Crimes don't always happen in the dark, but they do with greater frequency. It's important to take care when walking at night in Central America, especially when you're alone (and that doubles when you're a woman). Particularly, walking alone at night in urban areas, but also certain touristy areas, beach areas, and even rural areas. It's helpful to research the areas you're visiting, or to ask around once you're there. If there's any question, take a taxi, or stick in one spot.
Be careful swimming in the ocean. First, because of riptides – some beaches in Central America are very dangerous. Do your research. Second, because you have to leave your stuff on the beach to swim, and it's a prime opportunity for being robbed.
Buses. Central America buses range from first-class luxury buses to chicken buses packed with locals, often standing shoulder-to-shoulder in the aisles. With them come a number of safety concerns – like the bus driver's driving skills, which you can't do much about. However, you can keep a close eye on your stuff, as public buses are a favorite destination for petty thieves. Don't put anything important in your pockets, ever. Bus rides are the best time to dig out that underclothes money belt. Keep your daypack or bag on your lap, or if you're standing in the aisle, wear it in front with your arms over it. But keep your passport accessible – many times, I've had a police officer or soldier climb onboard and ask to see it.
Be wary of swindlers. I couldn't come up with a name for these folks, but I'm talking about cab drivers, tour operators, and the like who aggressively descend upon travelers when they arrive at many destinations. They're always touting something, and they often outright lie (like telling you the hotel where you have a reservation is unsafe) – usually because they get a kickback from another hotel or attraction. They're enraging, and take a lot of energy to deal with, even when you haven't just endured a 6-hour bus ride. I try to (politely, though not always) ignore them; when seeking a cab, I head for one that isn't hassling me, even if I have to hail one on the street. Trust your internet research and your judgment, not what someone with an ulterior motive tells you.
Watch your bag. Purse, daypack, whatever it is you travel with when your luggage is safe in your hotel room. Don't keep anything too valuable in it. If it's a purse or bag with a strap, wear the strap across your chest so it's harder to yank off you. In crowded places, wear your daypack in front – they're super-easy to pickpocket.
Consider a throw-down wallet. Many travelers I know keep their day's cash and an expired card or two in the wallet they keep in their pockets or bags. If held up or pickpocketed, they won't lose anything too important.
Consider a money belt. Underclothes money belts are not the most comfortable things, and I've worn them less frequently over the years I've traveled in Central America. However, they're great for certain situations – like overnight buses, or crowded marketplaces, or situations where you need to keep your passport on you, but don't feel comfortable keeping it in your purse or daypack.
Take special care when driving. Driving is a lot more aggressive in Central America. If you're renting a car, make sure to research (and follow!) all local traffic laws, and drive as defensively as possible. Try to avoid driving at night, if you can. Many roads in Central America are unpaved or in bad condition; rent a car or SUV with four-wheel-drive if you're driving outside the cities.
Taxi safety. The color and name of official taxi cabs varies by city and country, but always make sure the Central America cab you're taking is licensed and official, especially airport cabs (which often have a set rate). Even better, ask your hotel to call one from you. If you're traveling within a city, ask for the meter ("el metro") to be turned on. If you're traveling a longer distance, or in a cab without a meter, agree on the price before you climb in. It doesn't hurt to write it down. Lastly, tipping isn't necessary when taking Central America cabs.
Keep expensive things put away. That goes for iPhones, Blackberries, other cell phones, Kindles, laptops, so on and so forth. Don't even play with them in cabs or on public buses (though I do read my Kindle on Ticabus, for example). Only take them out in your hotel, or in other safe spaces – unless you can afford to have them stolen (and possibly, the anxiety of being mugged). Be discreet with cameras.
Don't photograph children. Don't photograph children without permission from their parents or guardian, no matter how cute the kids are. This can be very serious. There are fear-fed rumors in certain areas of Central America, especially in rural Guatemala, of travelers kidnapping children. A decade ago, a Japanese tourist was killed by a mob for photographing a Guatemalan child.