You can top-up, or add value, at any 7/11, or by using a top-up machine. These top-up machines are everywhere, including inside most BTS stations on the ticketing mezzanine. You can do up to 12 individual top-ups, at 12 baht each (10 baht goes to the balance, 2 baht is the fee), and each will extend the validity for 1 month/30 days, up to a maximum of 365 days.
http://www.entrepreneur.com.ph/image...ine/u_load.png
There are a gazillion other ways to add value: ATM, credit card, True service kiosk, TrueMoney...
http://truemoveh.truecorp.co.th/3g/topup?ln=en
TrueMove generally does not sell pre-paid SIMs in their retail outlets, preferring to outsource this low-volume activity to phone shops and their corporate "brother" 7/11. Terminal 21 is a bit of a tourist haven so expect to get gouged a bit as their rents are quite high. You can buy any SIM, add value, then subscribe to the services you require. A lot of the "promotional" SIMs are targeted at specific groups.
Once you add value you can subscribe to a monthly, recurring data plan, if you want.
I usually recommend the 4G plans:
http://truemoveh.truecorp.co.th/4g/s...aid/entry/3340
You can also subscribe to a plan in a 7/11 as they have a rack with all packages.
http://truemoveh.truecorp.co.th/3g/t...try/3559?ln=en