my wife takes one pair of comfy sandals and a simple pair of slightly more dressy shoes for evenings...

you will need to cover your shoulders in some temples...anything royal in thailand you will need a skirt or long slacks and sleeves on your top, even if just a cap sleeve.

no shorts in royal thai places either. men or women..

cambodia and vn are much more relaxed...

its best to go dressed in a fitting way...

my wife carries a light weight scarf in her purse that she can throw around her shoulders...

remember all natural loose fitting clothing, mostly cottons and/or linens...

To add to what rhkkmk has said alrady. For the official places and significant Wats in Thailand you should follow the following guideline(from the Tourism Authority of Thailand):

Dress Code: Visitors are required to dress appropriately. Thus the following dress - code (applicable to both ladies and gentlemen) is requested:
1. Shorts, mini-skirts, short skirts, tight fitting trousers, as well as tights can not be worn as outer garments.
2. See-through shirts and blouses, as well as culottes or quarter length trousers can not be worn.
3. Sleeveless shirts or vests can not be worn as outer garments.
4. Sandals (without ankle or heel straps) can not be worn.
5. All shirt sleeves, whether long or short, can not be rolled up.
6. Sweat shirts and sweat pants, wind-cheaters, pajamas and fisherman trousers can not be worn.

For the malls, shopping walking about kind of places you can pretty much dress how you like. Be aware that some hotels do not allow the wearing shorts or sandals in their restaurants or hotel itself.

This is a useful website for visiting Thai palaces: http://www.palaces.thai.net/day/index_gp.htm

While I prefer to go sleeveless myself, I found that on a recent trip to Turkey my shoulders constantly burned despite lots of sunscreen. Therefore, in Thailand and Cambodia I wore short sleeves to cover up a bit more and found it was better to have added protection. No matter what, you will sweat! I wore lightweight capri pants in both Thailand and Cambodia and found them acceptable everywhere. You can wear whatever you wish at the Cambodia temples. Many people wear open sandals although I prefer sneakers as they give me more support.

Worse comes to worst.
In Bangkok, if you show up at the front gate of Royal Palace improperly dressed, there is a booth near the entry that can provide clothes to cover you up properly.
You must leave your passport or credit card as security though.

A couple of general principles:
having more exposed skin will make you hotter and more prone to sunburn
tank tops and shorts are considered resort/beach wear in SE Asia and not appropriate for city wear.

To enter temples and royal properties in Thailand, you must have at least short sleeves (no sleeveless or tank tops) and long pants. Capris or cropped pants are allowed by some GP guards and not others - the signs clearly state that they are not allowed. No shorts are allowed in the lobby, restaurants or bars at the Oriental Hotel or at the Skybar or other similar places. Some tourists choose to wear shorts or sleeveless tops in Bangkok as they shop etc. You won't be refused service, but you may well be seen as rather clueless.

In Cambodia, the temples at Angkor are not active temples (though many have Buddha images that are tended by monks or nuns), so the dress codes are not as stringent. However, it is incredibly hot in Cambodia and you'll want to make sure to protect yourself from the sun as much as possible. Also, you'll want to protect your toes. The stairs at the temples are very steep, and there are many fallen stones. I wore Merrills, but Keens would also work well. I saw a very uncomfortable French woman and her daughter who were wearing flimsy flip-flops. You want some thing with some support that will protect your feet.

Tank tops are not considered appropriate city wear for either men or women. One of the most uncomfortable travelers I've seen was a German woman in Hanoi siting in a casual cafe wearing a tank top. She was stared at by all of the Vietnamese, though they didn't say anything to her. You will be more comfortable and the locals will be too if you dress more appropriately. A sleeveless dress for a woman would be ok many places (but not temples), as long as it isn't spaghetti or lingerie-strap type.

You may also consider the fact that the mosquitoes can be nasty and you will be happy to have more skin covered than not.

such helpful tips. thank you. i don't usually wear sleeveless, but thought i'd sacrifice because of the heat... hadn't thought about the actual protection from heat offered by sleeves. will have to rethink the wardrobe. i do tend to use an umbrella for the sun (rather than a hat) and hope that's seen as fairly normal in asia. the carrying-a-scarf idea is a good one and i'll try to do that too. thanks again. kawh

An umbrella is fine. Monks with umbrellas are a very common sight. (Shaven heads of course.) Personally I prefer a hat as some of the outer temples such as Beng Melea involve scrambling and climbing over rocks.

"Some tourists choose to wear shorts or sleeveless tops in Bangkok as they shop etc. You won't be refused service, but you may well be seen as rather clueless."

We have been here before and the above quote is not correct. A lot of Thais wear shorts themselves, or at least cargo pants. Many ladies wear shorts. You don't wear inappropriate clothing when visiting wats, museums, places or when doing any official business. Shorts around the street are fine and are commonly worn.

Apart from Temples and other such venues, cover up against mozzies and sun.
I keep on meeting people complaining about mozzie bites and there they are sitting in a bar/restaurant in the evening in shorts or sleeveless tops - som nam na!