What are the best locations and tips for extending a Tourist Visa in Thailand?

We don't do those "what are your experiences" posts but I'll let this on to give you answers 😛 🙂

You can apply for an extension at the immigration offices in Bangkok (or Chiang Mai) when you have 45 days or less left on your entry stamp.

There is no up side to waiting as they add the 30 days to the expiration date you have now.

I'd say there is no difference between the two as far as the process,

you need a TM30 to get a 30 day extension on a TR60 entry stamp at Section J in Chaengwattana immigrations

AND

You need a TM30 to get a 30 day extension on a TR60 entry stamp at either the main immigration office OR Central Festival Mall in Chiang Mai.

About the only difference is Chaengwattana is busier than Chiang Mai by about a factor of 10 or more on ANY day you go there,

Where as you could probably skate into Central Festival with the copies, form, photo, TM30 and get the extension pretty fast

You have to decide what office you're going to go to.

This is the documentation you are likely to be asked for at either location