What are the best strategies for prolonging a Multiple Entry Visa stay in Thailand through border runs and extensions?

It depends, if a border run makes more sense (if you are close to a border anyway) do a border run, if such a border run takes a hell of lot of time (think nine hours from Bangkok) then extending is a much more useful thing to do, and cheaper. As long as you make your final entry just before the use before date, you can still maximize the visa by doing a final extension. I disagree with Tod, extending can save massive amounts of time, an hour at immigration plus the odd hour in the taxi, as opposed to 9 hours for the nearest cambo border point from Bangkok. Not to mention that the Cambo visa cost money + transportation cost, I am sure an extension is actually cheaper.

I have found that you gain NO time at all extending the 60 day stamps by 30 days each time. All you do is burn up the validity of the visa itself and spend 1900baht.

Every 60 days you border-bounce to get another 60 day stamp making sure your last border bounce is on or before the "enter before" date on the visa itself. That takes you to almost 8 months and you extend that entry by 30 days to get you to the ninth month ;)

Leo's advice is right on. However, you don't have to do the 30 day extensions at immigration if you don't want to. You can just pop across a border after 60 days if you are near one. I hate spending half a day at immigration so I prefer to border run.

You can extend the initial 60-day entry 30 days at any immigration office for 1900 baht. Then do a border hop for another 60-day entry and repeat. You can even do a border hop on the day before the visa expires and get another 60+30, which means you get nearly 9 months out of the 6-month METV.