Bring as little as possible - Medium sized suitcase, one medium-sized shoulder bag, duffel bag or backpack should do. Pack lightweight clothes, unless you're going to be in the North in the cool season, in which case you should have a pullover. Natural fibres can be cool and comfortable, except when they get soaked with sweat or rain, in which case they quickly become heavy and block air flow.
Sunglasses are a must for most people and can be bought cheaply in Bangkok and most provincial capitals. Slip-on shoes or sandles are highly recommended - besides being cooler than lace-up shoes, they are easily removed before entering a Thai home or temple. A small torch (flashlight) is a good idea, as it makes it easier to find your way back to your bungalow at night if you are staying at the beach or at a remote guest-house. A few other handy things include a compass, a plastic lighter for lighting candles and mosquito coils (lighters, candles and 'mossie' coils are available in Thailand) and foam ear plugs for noisy nights.
Toothpaste, soap and most other toiletries can be purchased anywhere in Thailand. Sun block and mosquito repellent (except high-percentage DEET) are available, although they can be expensive and the quality of both is generally substandard. If you plan to wash your own clothes, bring along a universal sink plug, a few plastic clothes pegs and three metres of plastic coed or plastic hangers for hanging wet clothes out to dry.
If you plan to spend a great deal of time in one or more of Thailand's beach areas, you might want to bring your own snorkel and mask. This would save you having to rent such gear and would also assure a proper fit. Shoes designed for water sports, eg Aquasocks, are great for wearing in the water wheater you're diving or not. They protect your feet from coral cuts, which easily become infected.