@Mary *******
Chiang Mai allows you to apply for a new extension of stay when you have 45 days or less left on your current one.
I know for a fact IF you show up at the immigration office showing you are leaving the country before your current extension expires and show them that you will not be back until after the extension expires you can get a new yearly extension pushed thru.
It takes refusing to take "cannot" as an answer from some low level, paper pushing, passport stamping immigration official :O <- by that I mean you need to be willing to push it up the chain of command as high as it takes to get someone to allow you to do it
BUT
I know it can be done because I was able to assist a friend in getting her yearly extension 86 days early ;)
otherwise, I'd say getting the year-long, multi-entry Non-O-A visa from one of the 4 thai consulates in the US that issue them before you come back would be the way to go .