Clothing/Shoes/Weather Gear: if you plan to visit thailand in general and bangkok in particular, during the rainy season in thailand (usually starts from june to october) like we did in october this year (2012). then prepare to bring a small and lightweight portable umbrella that you can neatly fold for easy storage on you bag or just by them at any of the markets, night markets, convenience stores and malls around the bangkok area and these small and portable umbrellas are very cheap and you can buy them for only 150 baht or less. at least the showers here are relatively mild as compared to the typhoon visited areas of south east asia like the philippines or vietnam or cambodia. or you can just buy the water proofed plastic overalls which are disposable at any convenience stores or markets too (costing 100 baht per piece) but the downside of these water proofed overalls is that they are bulky when you remove them off and hold them after the rains stop. you're best bet is still the small and foldable umbrellas.
Luggage and bags:
Umbrella for the rain season
First Photo: Raining in Suan Saranrom Park
Second photo: Raining in Suan Saranrom Park
Third photo: Raining in Chao Praya river. Inside the boat
Forth photo: Raining outside the Wat Phra Khaew
Miscellaneous: I was in Bangkok in October during the rainy season. Good idea to bring an umbrella along so the weather will not halt you in your path. Maybe wear shorts and sandals so you won't get wet if you are planning to going to any jetties. Some jetties are known to flood during this season as river water flowing from the north overflows. The weather is not too cold (definitely above 20degC) so it's quite pleasant still.
Luggage and bags:
August is still monsoon season in Thailand, Nevertheless, I never found really heavy rains, just some cloudy days and a few raindrops some evening.
But, just in case, U better bring a portable umbrella and keep it with you. Plastic anoraks are not advisable, as you might sweat a lot (high hummidity here).
Miscellaneous: Bring an Umbrella or buy it from there, Bangkok is notorious for sudden rain and thunder strom without warning.