"You'll once a year need an balance confirmation letter from your bank together with a 3-month statement (typically 200 baht cost, valid for 7 days) and copy of (all pages) on your bankbook, which also need to be updated, often same day or day before applying for extension of stay."
Procedure may be different, depending on the Immigration Office.
At Jomtien, if you have the 800 000 baht in an FDA. the procedure is as follows:
1. Deposit 100 baht in the FDA.
2. Request the bank to update your bank book.
3. Request a letter stating that you are the holder of the account and how much is in it. (Cost: 200 baht.)
4. Request a 12-month statement. (Cost: 200 baht.)
5. If you are applying for an Extension of Stay yourself, you will need to copy every page of your bank book.
6. If you are using an agent #5 is not necessary.
Source: Recent personal experience.