There are a million lists in travel guides and online of how you should prepare for a trip. The biggest thing that I wish I had done was to get a motorcycle driver's license and the International Driver's License to go with it. In many places in southeast Asia, renting a 100-125cc motorcycle will cost you $5US/day, and it's often a very practical way of getting around. It can also get you to places that no guides will take you (for less than $70, anyway). You don't actually need a license to rent a motorcycle here, but I'd guess that your health insurance (you got health insurance, right?) would evaporate if you were riding a motorcycle without it and you had an accident. Besides, the training would be a good thing.
Two items I don't think are emphasized enough: bring several plastic bags, and a Microlite flashlight or equivalent. Plastic bags weigh virtually nothing and are essential for wrapping liquids you have to carry in your backpack. Think of shampoo, and especially insect repellant: DEET eats synthetic fibres, so you can say goodbye to your backpack if it leaks. The Microlite also weighs nothing: we're talking about an item the size of a quarter that weighs six grams. And you will use it more than you ever imagined - in power outages (you will encounter several), in towns that have no power at all (you will encounter at least one), in caves, to find things in your backpack on the bus at night. You will use it. In fact I have two of them in case the first one fails or gets lost. If you're willing to carry the weight of a more powerful flashlight, you won't regret that either.
I recommend Mountain Equipment Co-op's products, especially their backpacks. No, this isn't a paid product placement, I just like their gear. My mid-sized backpack is well designed, durable, and cost $40CAN. I also highly recommend moisture-wicking and/or quick-dry clothing: MEC's line is fine, but I like Reebok's shirts better (when I can find them) and Misty Mountain's pants are good.