There are different ways to archive this.
I my self always buy a 1 year SIM from AIS, DTAC or True (DTAC, Ttrue are now merged, but still use a seperate net, this maybe changed in the future. Now, you also can add a DTAC number in the True Service app).
- 1 Year Sim, afterwards, refill with enough money for another 1 year plan (min 12 refills) or buy another new 1 year SIM.
- buy a plan only when needed, but then you have to take care, that you always have enough validity.
As you told your card run out fast, then I would say you should buy DTAC or True, as there you can extend your validity of the SIM for 30 days for 2 Baht. Normaly you refill for extending, but if you not use the card that much, then you will have after some time much to much money on it... so better then to pay for example 12 Baht for 6 month extension (*113*180*9# DTAC)
I use the 1 year plan, because I also use this router, when I go on short trips arround Thailand. Then I plug in my router and have my internet to use with tablets instead to use mobile sharing. Mostly the Router has a slightly better signal than the mobile.