Street food, again and again :
Like always : to eat cheap, you have to eat local so go to the street food stands. Especially in Thailand, where it's not in the local custom to cook at home (most don't even have a kitchen). You can eat well between 0.50 € and 2 €.
These small shops (which I will always associate with Japan to have discovered there), offer a little bit of everything especially in food. Some products are more expensive, but others not like some foods and beverages such as beer.
Moving away from "popular areas" :
To sleep, move away from popular quarters. No need to go for miles either, but the prices drop as soon as there are a few streets away, and often the guesthouses are much nicer.
Always have cash :
Always have change on you, this will avoid scams like "oops I can't give you change" especially with taxis.
Withdraw « large amount of money » :
If you withdraw a larger amount than a simple 20-50 €, the commission of 150 to 200 bahts becomes more negligible.
Become a Ninja :
More than once the prices shown were not what we had seen, and we often found it excessive. Take time to bypass the monuments. You will quickly discover that there are often unsupervised entrances that will allow you to visit for free.
Be careful with taxis :
Be vigilant in public transport including taxis. We didn't have too many worries, because although they tried more than once, we always get things clear.
Read also : Thai Taxi Survival Guide.
Go with Uber :
Even here Uber meets its success, especially in Bangkok. This is probably the most economical way to connect the airport to the city.
Travel by train :
We agree it's a bit slow, and not the most cozy, but it's the cheapest, and especially from our point of view one of the coolest means of transport to enjoy both landscapes and part of local life.
Night buses :
It's also a very good option since you have transportation and accommodation at the same time. This makes a small saving on accommodation. Even VIP buses remain economical and offer meals (dinner and a light breakfast of the times).