Travel advice for those planning a trip to Southeast Asia

1. Packing before you go: once you're done packing, unpack half of your clothes. Laundry generally costs $1/kg. They will wash it and fold it for you. You only need enough clothes to last you for a week, at the most. Clothes are so cheap there you can literally buy tees, socks, and underwear and throw them away as you go. I met people who were actually doing this. You don't need to go that extreme, but I'm just trying to highlight how cheap clothes (and laundry) is in Southeast Asia.
2. You don't need jeans. I wore them once in Hong Kong, and that was it. Maybe you will need nice pants to go clubbing in Hong Kong, Saigon, or Bangkok, but I was able to wear shorts literally to every bar I went to (and I was not out of place). Only bring nice pants or jeans if you are really into clubbing.
3. In theory you could do your entire trip with a good pair of sports sandals. You don't need to bring running shoes, and dress shoes are really unnecessary. If you plan on doing a hardcore trek then hiking books may be a good idea, but most treks can be done in sports sandals. In any event, you can buy any sort of shoe/sandal you may need for really cheap in Asia. Just bring one pair of shoes/sandals with you and buy anything else on arrival.
4. Bring a small (swiss army) knife. Just remember to pack it in your checked luggage.
5. Bring duct tape. Wrap a bunch of it around an old piece of I.D. that you don't need anymore (make sure it doesn't have sensitive information on it in case you lose it). This way it's more compact than if you bring an entire roll.
6. Don't go crazy with the toiletries. You can get pretty much everything for cheaper where you're going. If you have important medicines then buy that at home in case you are worried about counterfeit pharmaceuticals.
7. Buy toilet paper. Keep it in your day-bag. Trust me. Whether at a toilet in a smaller city, or in the middle of nowhere when your bus parks at a rest stop, you cannot rely on every bathroom having toilet paper.
8. Buy your bug spray at home. It's hard to find bug spray in Asia with more than 20% deet (generally it's recommended to have 30% or more deet).
9. If you're buying a new camera, get one that is waterproof and that has a quick shutter speed. There are many pictures I missed out on because it takes a couple seconds for my camera to snap pictures. Also, there are some pictures I don't have yet because I left mine in my bag and relied on travel companions that had waterproof cameras. I may not get those pictures for a few weeks/months, and some I may never get.
10. Sometimes travel agents sell flights for cheaper. I got a flight from Siem Reap to Luang Prubang for $100 less than I could find it online. It never hurts to check... although it's usually cheaper to buy flights online.
11. If you're traveling during low season, never ever books hotels or hostels online. It's possible some places will have no vacancy, but there will always be vacancy somewhere, and you will almost always be able to bargain them down. There is no bargaining when you book online! My experience was that you can bargain a hotel down anywhere from 10-33%. If they don't budge, then just go somewhere else down the street. I would only make an exception if you are arriving somewhere very late at night or early in the morning, and you know you'll be really tired and just need a place to sleep. Always ask to see a room before you start negotiating price. Don't be shy to ask for another room that is more quiet, with a better view, on a higher/lower floor, etc.
    
12. Even meals at proper restaurants can be bargained for sometimes. In Angkor Wat my friend and I got a discount on our lunch that came out to about 20%. The lesson: it never hurts to ask.
13. Bring Immodium. It saved me on numerous occasions.
14. You only need to bring one or two books. Book exchanges and used bookstores proliferate, so it's easy (and often free) to turn one good book into another. Alternatively you can just ask other travelers as you meet them if they have a book they'd like to trade with you.
15. Guide books are overrated. Usually their maps suck, and often the writer hasn't visited everywhere they've written about. For restaurants, just go wherever you see the most locals. If a place has only tourists it's probably more expensive, and not true local fare. If you come across a map that looks really good and costs a couple dollars, it is definitely worth it to buy. The Nancy Chandler map of Bangkok jumps to mind as one example of a really good map. A quick look at hostelbookers.com, www.tripadvisor.com, or even lonelyplanet.com before you visit a city will give you an idea of where to stay, where to eat, and what to do. Personally, I found recommendations of other travelers to be more reliable than guide books.
16. When you see people on the street selling fruit, they will cut it for you when you buy it from them (I realized this fairly quickly, but other people I met were surprised when I told them this - they didn't realize vendors would cut the fruit for you at no additional charge).

That's all I have for now. I'll probably add to this later next time I sit down to blog.

Update (May 2012): I will never get some of my pictures from the Kuang Si Waterfalls near Luang Prubang because my friend lost his camera before he got to send me the pictures. One more reason to have a waterproof camera yourself!