For doing my visa I still have to go to Bangkok. Though Korat already has its own Immigration Office, for one they don't have the personnel yet to do visa extensions, just 90-days reports, which I don't think I have to do at all. Nobody ever asked me for it in 17 years.
So I have to go to Bangkok twice a year, once for submitting my application, four weeks later for picking up the stamp in my passport. For two I have to go to the embassy to get a paper for my application anyway.Since I'm married to a Thai and entered Thailand with a "Non-Immigrant Class 'O'-Visa". I can get the extension for one year at a time. My wife has to come along and sign a paper - I guess some kind of guarantee.
When I'm finished I always take an empty form home so I don't have to stand there and fill the application out in a hurry with undecipherable handwriting; I fill it out at home with a typewriter or nicely in block letters. Now with the possibility of downloading the application form from their website http://www.immigration.go.th/nov2004/en/base.php I download it into my PC, fill it out on the PC with MS Office Word, and print it out both sides on the same sheet. BTW on that website the procedure and details as to what kind of visas are available are described in detail along with other important visa-connected regulations.
Apart from all the personal data on the application form they ask only for the reasons for extension. Very simple answer: "I am married to a Thai", no excursion into how much I love Thailand.
On the back of the application they ask for a picture (4 x 6 cm). They want a newer picture, I just have to make sure that it doesn't look like the one I gave them last year. Because they always look at my application from the year before. If it's the same picture, I gotta run to get a new one done.
Together with the application I hand in a certificate I obtained from my embassy upon having showed them the paper about my pension. The embassy guarantees on there that I'm receiving monthly annuity payments and tells about how much in Thai Baht. The embassy charges me about 1.200 Baht for that.
Then I submit a certificate from my Thai bank, stating what transfers I received from Germany within the last twelve months and how much the current balance is. They charge 200 Baht for that.
Making a lot of photocopies already at home with scanner and printer is also good. I can make them look nice and don't have to wait at a photocopy machine. In fact, I just print out PDF files of my passport, marriage certificate, my wife's ID card, her house paper and my bankbook. Every photocopy has to be signed by me to guarantee its truth.
From the passport they ask for copies of the page with the personal data, the visa stamp, last extension stamp, the white TM.6 form that was stapled into my passport at last entry and the stamp of first entry with that visa and from last re-entry. From the bankbook the first page with account number and my name and the last page with the current balance. Last time I paid 1.900 Baht fee. Being prepared it's a very fast in-and-out procedure. They even say thank you to me - for making it easy for them.
There are lots of different and often contradicting posts on forums as to how much you must have on your bankbook or how much the certificate from your embassy must show as your annual pension; I don't want to add to that confusion and ask everybody to find out for himself by asking at the immigration office or looking up a really reliable source. What is one I also cannot recommend. Too much contradiction and confusion. Think a minimum amount is only high for people not related to Thais and in any case only the first time.
Upon submitting my complete application I get a temporary stamp for 28 days and am told to come back then to find out about the result. They have organised the whole procedure a lot better and more transparent, so I don't have to wait and run around so much anymore. New is at Soi Suan Phlu Office the giving out numbers, so you don't have to sit and wait for your turn but can do something else and come back when your number is coming up.
As to the Korat Immigration Office, the requirement for getting the income certificate from the embassy makes me go to Bangkopk anyway. so I for myself am not really waiting for it to make visa extensions, unless the German government opens a consulate here as well
Let me try to shed some light at the risk of boring those who don't want to know. 
