Book an appointment to save waiting in line and make sure you print out two copies of the appointment, to hand the IMO on arrival.
Make sure your application is in blue ink and not black, otherwise you will have to rewrite it, which is what happened to me.
This year they wanted the usual updated bankbook, a letter from my bank, but they also wanted a statement of my account, eventhough it was an invested account....no if's or buts!
I have been doing visa extensions for 20 years at Nonthaburi and not once have I been able to get through without something being wrong.
Next year I'm reverting back to an extension based on Retirement. The marriage visa is just too much of a hassle and twice the paperwork. The biggest draw back is the extra 400,000baht in the bank for the retirement extension.
Best of luck!