A quality pair of flip flops is hard to find among the massive piles of fake rubbish in the local markets. The wrong pair will break within weeks or make walking around into a painful chore. Havaianas are considered one of the best - but often faked. If you're in SEA already, buy them from a Havaianas store or Amazon. If none of these options are viable, don’t buy them at all as they will be fake. They’ll end up costing the same or more as you'd normally pay at home, so I'd suggest getting them before leaving.
Many tourists tend to go for the type of sandals that have a strap around the upper foot and around the back near the ankle. These are great for long walks on uneven terrain, but impractical for day to day life. Moving in and out of homes (and some shops) in SEA requires taking off and putting on your footwear because wearing footwear indoors is considered to be very impolite and brings in dirt from outside. So try to look for a shoe that is easy to take on and off.
Bring at most one pair of pants/jeans, one nice shirt (for partying, business meetings and weddings - if you’re single you will be invited to at least one), plenty of underwear and t-shirts. If your clothing size is XL or bra cup size greater than C, finding clothes that fit you in SEA is going to be difficult. Stock up before leaving so you'll be set for your whole trip.
Clothes like t-shirts and shorts are cheap in SEA so you can always buy more later.
Dress Like An Adult
Everyone around you is either in holiday mode as a tourist or making-do mode as a local (or don’t-give-a-toss-anymore mode as an expat). Because of this, the temptation will be to slack and not put too much effort into your appearance on a day to day basis. Asian cultures tend to place a bigger emphasis on appearance than in the west, so it's important to follow along with this cultural aspect. If you’re dressed like an adult instead of a 19 year old backpacker (e.g. singlet with beer logo) you’ll get much more respect. This translates into more helpful service from bureaucrats and shop assistants and less scam attempts from taxi drivers, for example. There’s no need to overdo it, just don’t stumble into the immigration office to extend your visa with a hangover, stains on your singlet and holes in your shorts.
It’s going to be very hot, but try to resist the temptation to walk around in public with your shirt off. Even if local men are sometimes doing it (they usually won’t), they’re at home and you’re the visitor.