I know this isnt too complicated, but its probably much easier explained to me by one of the experts here. We are arriving in Thailand from the USA spending a total of 89 days. So I was thinking of not getting a visa ahead of time but using the free 30 days to start with. Before the 30 days are up I thought we could go to Cambodia, as we like the beach there. Am I safe in assuming that we would need to fly there with already booked from home flight as proof of an onward ticket out of thailand within the first 30 days? Then I thought we would come back to Thailand by bus with around 40 days left in the trip. So we would then need to obtain a visa prior to returning to thailand, correct? If we were planning on leaving Thailand again before the next 30 days was up, would they let us come back in without proof of onward travel and without purchasing a visa? Best way to obtain the visa while in cambodia? Hope this made sense. This will be our third year in a row back to Thailand. You would think that I would have all this stuff figured out by now! Thanks for any help.

There is a good chance that the airline from USA will not let you on the flight without a proper visa.

Just read your post again , and yes you would need the onward flight within 30 days.

I would obtain two entry tourist visas and use them. They are not that expensive and will avoid any problems (just avoiding the need to visit Embassy in Cambodia would be reason enough for me - although it is reported better than previously).