With only two seasons to worry about (mostly), Southeast Asia doesn't require too much baggage space to pack for.
When planning a trip through Southeast Asia's top tourist sites, you mainly need to pack light, loose cotton clothing; you can't go wrong with these for most destinations in Southeast Asia, all year round. You also need to be mindful of the local culture: pack clothes that cover your shoulders and legs when visiting Buddhist temples, Muslim mosques, or Christian churches.
Everything else depends on where – and when – you go.
Packing for the Season: Summer or Monsoon?
Between April to May, most of Southeast Asia tends to be hot and dry. From the end of May to October, the monsoons arrive and the climate gets extremely rainy and humid. The rains give way to cool and dry winds blowing from the north from November to February.
Most places in Southeast Asia generally follow these three seasons. Read up on the local weather to find out what the climate's like where you're going, and pack accordingly.
Traveling during Southeast Asia's monsoon season? Avoid packing that heavy parka, which might be too warm for the humid tropics. Instead, bring sandals, a light waterproof raincoat, and a portable umbrella. More information here: What to Pack for Monsoon Season Travel in Southeast Asia.
Going during the summer months? Bring a hat and sunglasses to ward off heatstroke. Bring light cotton clothes, sandals, and flip-flops.
Alternatively, you can simply buy your clothes at your destination, if you're staying in or near the cities. More information here: Pack UV-Resistant Clothes For Your Southeast Asia Trip.
Going during the cool months? Bring warm clothing - warmer if you're headed to higher elevations. A sweater might do in Bangkok in January, but may not be warm enough for the mountainous North.
Packing for the Location: City, Beach, or Mountains?
Cities – especially Southeast Asian ones close to the equator – are notorious heat sinks. In urban areas, cool seasons tend to be less cool, and hot summer months can be positively hellish. Light cotton clothing ought to see you through.
Most cities in Southeast Asia have places that sell really cheap clothing, so you might consider packing very light and buying your clothes at your destination instead! (Important Tip: if you're exceptionally tall or broad, this might be a bad idea, as the clothes sold at such places are made to fit smaller Asian body shapes.)
Beaches may enjoy fresh breezes blowing in from the sea, but they offer little protection from the sun. Apart from the summer clothes mentioned in the previous section, bring or buy a towel, flip-flops, and a sarong. (The sarong is the Swiss Army Knife of clothing. Wear it to the shower to deter peeping toms! Use it as a makeshift blanket, bedsheet, sunshade, or curtain! Use it in lieu of a towel! The possibilities are endless.)
Higher elevations tend to be cool in the summertime and positively frigid in the cold months. Bring warmer clothing, like a sweater or a fleece jacket, if you're headed to places like the Cameron Highlands in Malaysia or trekking up the region's many mountains or volcanoes.
Supplement this with a flannel blanket.
How can you protect against the sun in the searing tropics? Read this:Sunburn & Sun Protection Tips in Southeast Asia.
Packing Essential Odds and Ends
Travel Documents: Protect your important travel documents from theft. Copy them in triplicate: passports, driver's licenses, airline tickets, and traveler's cheques. Staple the photocopies together and pack each copy in separate locations.
Keep the originals in a secure location, like a hotel safety deposit box. Alternatively, you can scan your documents and keep the files in an online storage service, for easy printing when you need them.
Pharmaceuticals and Toiletries: Pharmacies in urban areas can provide all your day-to-day stuff - shower gel, suntan lotion, deodorant, toothbrush and toothpaste, and shampoo.
While medical supplies are also easy to find in cities, you may want to be absolutely sure and pack your own – antacids, rehydration sachets, anti-diarrhea pills, analgesics.
If you're bringing prescription drugs, bring the prescription too. Keep your insurance number handy, just in case.
Bring toilet paper for the eventual emergency, and soap or anti-bacterial gel for use afterwards.
Don't forget sunscreen and mosquito repellent. Leave them behind at your own peril.
Electronics: Electrical systems in most Southeast Asian countries use different voltages. Bring a transformer or adapter if your electronics don't play nice with the local electricity. Bring extra batteries and film, in case you go someplace where you can't buy replacement stocks.
Extra Luggage: always a good idea, especially if you're bringing back more stuff than you came in with. This writer likes to carry a foldable backpack that takes up minimal space when not needed.
More stuff: You might want to bring one or more of the following items, if you find yourself some way off from the beaten track. If you're hitting the hiking trails, please read this page to see what else you might be missing: Packing Tips for your Southeast Asia Hiking Trip.