Although it is getting more and more lax in some areas, Thailand is still quite conservative. In Bangkok you can wear shorts and spaghetti strap tank tops in the main tourist areas, but otherwise you should keep that kind of dress for the beaches. In temples you should always cover your arms and legs (women in particular). It's not mandatory in all wats like it used to be but you should be respectful and cover up. Even when in town in beach areas, you CAN wear skimpy clothes but you won't see the locals doing it. In some areas (smaller towns) the locals still swim in shorts and a shirt so keep an eye on your surroundings and don't be the only one in a tiny bikini. You'll want at least one long-sleeve item for temples, to protect from the sun, and in the evening for mosquitoes.