Reclaiming tax is comparatively easy. I go to the tax office in the city hall complex, which is sort of opposite 700 Year Stadium (your tax office may be different, depending on where in CM you live. Early in the year (March or preferably before), ask your bank for a document certifying the amount of interest and take this with you along with the bank book, your passport etc. Best to take a Thai speaker along also unless you speak good Thai. I have always found the people there very helpful. Then you can apply for a tax ID. Actually reclaiming the tax is mostly done over the internet now, but I used to go there and fill in the forms on paper. I have always received tax back in a month or so after applying.