I know there's a few threads out there with various ways to keep your Thai SIM topped up and 'live' between trips to Thailand. I just came across a new way (to me) but I'm not sure if info on this method has been posted before or not.
I found when I returned to UK from last visit the top up I made before returning only kept it live for another month. Bf topped it up again remotely in TL since I got back to keep it topped until June when I go back to TL. I used it to make an sms to bf in Thailand from UK today and decided to check the balance after, on getting that balance from DTAC/Happy by dialling *101# i got a further sms which i have'nt had before with an online DTAC address to top up. Just used it to top up Bt200 on DTAC/Happy using a debit card (but you can use credit cards) for ┬г4.62= Ex Rate Bt43.28 to ┬г. It's good until 23 July. The main DTAC site is in Thai but i used google translation when topping up...three easy steps it said....and was. The secure payment section had english and Japanese as well as Thai so no problems there. There were top-up options of Bt50/100/200/300/500.
Just another alternative to top up between trips that I wasn't previously aware of. Email- [www.dtac.co.th/irefill]