I'm traveling overseas and the country I'll be staying in (Thailand) requires proof of an exit flight. I'm not sure how long I'll stay there (I have the ability to stay a couple months to a year or longer) so setting the return date is difficult.
Anyone have suggestions on how to keep my return flight somewhat flexible?
You need to contact the specific airline that you have your flight with, or speak the agent that you booked it through.
As flyingbob mentions, talking to your agent or the airline is probably the most helpful. The reason being that they'll know what kind of ticket you are on, and how many date changes you can make, at what cost. For a trip that stretches over a long period, it's smartest in my opinion to go for a fairly flexible ticket, even if it is a bit more expensive. I've been on trips where I changed my mind 6 or 7 times, and I sure was glad I didn't have to pay for each date change then!
I've been travelling to Thailand for 25 years and never once been asked for proof of an exit flight. This visit I came with just a one way ticket and the Immigration Officer asked me of my plans and I said I might go to Malaysia after 2 or 3 months and that satisfied him. If you overstay it will cost you 500 baht each day, which they are happy to receive, so your problem. Not their's.
Bangkok is full of travel agents. If tourists did not book short and long haul flights with them they'd go out of business since the locals do not travel much