-- check the voltage, frequency, and plug type of the country you're visiting before you leave. This one is really a no-brainer; you don't want to go on a three-month backpacking trip across Europe with U.S. plugs on all your power adapters.
-- check your power adapter for its acceptable range of voltages and frequencies. In recent years, most higher-end electronic devices, including Macs, have come with "universal" power bricks that work in a voltage range between 100 - 240 Volts and a frequency range from 50 - 60 Hz. The power adapter for your device should have this information listed on it somewhere; on Apple adapters, it's usually on the "bottom" of the power brick, opposite the corner with the plug adapter. Apple's desktop models and peripherals like the Time Capsule have universal voltage as well, and those details should be printed on the exterior of the enclosure.
-- shop for plug adapters before you travel. You're going to save a lot of money and frustration if you shop around online from home. I don't know about the rest of the world, but I've seen stores here in New Zealand that charge anywhere from NZ$10 to $20 for a single plug adapter. In the U.S., you can buy an international travel kit from somewhere like Target for cheaper, but you can still do better than that; when I moved to New Zealand, I bought 15 universal-to-NZ plug adapters for about US$30 from dvdoverseas.com. I've been using them for about a year without any issues.
-- remember that "plug adapter" is not equal to "voltage converter". A plug adapter merely lets you plug your differently-shaped U.S. plug into a U.K. outlet, or what have you; it doesn't modulate the electrical input at all. For that, you'd need a voltage converter, a transformer in a box that will step down (or up) the voltage and frequency as necessary before it gets to your device.
Plug adapter - lets you insert tab A into slot B, and that's it
-- buy a voltage converter if your device's power adapter isn't universal voltage. If you have a device that only works with 110 Volts and 60 Hz, even if you've got a plug adapter that lets you plug your U.S. device into an outlet in Australia, the instant you do, it's ozone city. Unless your device's power adapter says "100 - 240 V, 50 - 60 Hz" somewhere on it, you need a voltage converter. All recently released desktop Macs, peripherals like the Time Capsule or Apple TV, and notebook or iPod power adapters should be universal voltage, as are many camera battery chargers. As far as game consoles go, the PlayStation 3's built-in power supply is universal voltage, but the XBox 360, the DS, and the Wii's power adapters are all specifically geared to the voltage and frequency of the country where they were purchased, so you will need a voltage converter for them. You can buy country-specific power adapters for those devices, but the expense adds up quickly; if you have multiple devices, it may be cheaper to simply buy a single voltage converter for all of them.
Voltage converter - for devices too big, dumb, or old to convert it themselves
-- buy the Apple World Travel Adapter Kit
if you need multiple plug adapters for just one country. At US$39 each, this is about the most uneconomical way to get adapters for your electronics, plus it only works with Apple's power adapters. Apple's adapter kit is great if you have just one device that you're taking to many countries, but if you have two Macs, two iPod power adapters, and a couple camera chargers, it's not the best option.
-- forget about wattage if you need to buy a voltage converter. If your device isn't universal voltage, a 100 Watt converter will do the job fine for most smaller devices if you only have one device to plug in, but if you have multiple devices, you'll need higher wattage. A guide I've always followed is to add together the wattage of all the electronics I'll need to plug into the converter and multiply it by 2 to account for power spikes (unless you're using a CRT television/monitor or anything with a big electric motor in it - then multiply by 4 or 5).
-- buy power adapters from travel shops. The price markup is insane.
Bottom line: Apple's products should work on just about any electrical system you encounter anywhere in the world, and the only additional purchase you'll need to make is a plug adapter. Other devices like camera chargers and game systems vary by manufacturer; always
check the info on the power brick before you plug it in overseas.