While living in European countries, I had a set of clothes for summer, and another set for winter. But I have only a set of clothes when living in Bangkok because it is hot and humid almost of the year in this city. If you ask me what to wear in Bangkok, I would say it depends on the places you are going.
General markets and shopping malls
Normally people do not wear traditional Thai dress in daily lives. We usually wear quite conservative clothes. No shirtless on when walking on streets, even if it is so warm on that day. Singlets/tank tops/short pants could be seen in markets, but could be rarely seen in shopping malls, which usually have cold air conditioning. Flip flops and open shoes are find for both markets and shopping malls.
Besides, it would be good to prepare an umbrella/ a rain jacket. The rainy season is around June to October, but you can also see rain in other months. Please bear in mind that torrential rain is common here.
Upmarket bars/restaurants and hotels
Sometimes in upmarket places the better you dress, the better you are treated. So, try to dress respectable; no singlets, tank tops, short pants, flop flops. For some places, you are not allowed to go inside with short pants.
Wat phra kaew and Grand palace has a strict dress code, while the other temples do not have such a strict dress code. Well, since we are Buddhist, temple is the place that we prefer you to dress conservatively. You can read what to wear when visiting temples in Thailand here.
Hope this information helps