I have never used an agent and although the process can be time-consuming and often very frustrating it is reasonably straightforward. Different offices have different requirements but as stated by others, go to your local immigration and ask all the questions you need to ask. I have always found the officers to be extremely helpful and professional. By explaining your particular situation I'm sure they'll be more than willing to advise you on what you need and how to succeed in your application. Remember, always smile and stay calm, even when presented with something that seems logical and unnecessary to you. At a total cost of 1,900B, why pay an agent up to 60,000B to do something you can learn to do yourself?

I recently renewed my Non O marriage visa in Korat for the fourth year. They didn't want my DTAC number from a recent visit to Vietnam but wanted my TM6 from three years ago connected to my now out of date, old UK passport (My new passport had all relevant stamps transferred into it by Korat Immigration) and the 90 entry certificate from my initial UK Thai Embassy application to retire here on a Non O marriage visa! The list gets bigger, better and longer every year. Never throw anything and always take everything is my motto.