Light cotton or other natural-fiber clothing is appropriate for Thailand; drip-dry is an especially good idea, because the tropical sun and high humidity encourage frequent changes of clothing. Avoid delicate fabrics, because you may have difficulty getting them laundered. A sweater is welcome on cool evenings or in overly air-conditioned restaurants, buses, and trains.
The paths leading to temples can be rough, so bring a sturdy pair of walking shoes. Slip-ons are preferable to lace-up shoes, as they must be removed before you enter shrines and temples.
Bring a hat and UV-protection sunglasses and use them. The tropical sun is powerful, and its effects long-lasting and painful.
Thailand has a huge range of clothing options at good prices, though it may be difficult to find the right sizes if you're not petite.