I do this pretty aggressively. I'm just now reaching the point where I have one of everything (except a stove), so pretty soon my spare change will be going towards replacing things I already have. That being said, I've gone through maybe ten pairs of shoes and boots, three backpacks, four jackets, and two sets of rain gear, plus endless baselayers, gloves, hats, bottles...
My secret has been getting in good with companies that have customer service centers that take back slightly loved items. I never, ever eat the cost of something that fails; if a seam rips, I'm getting a new one.
I bumped a $100 investment into a Marmot Super Mica on sale into a $250 Patagonia Super-Cell over two exchanges for no extra $. Woohoo!
That being said, I have bought things I'll never actually use that I should be selling on Gearswap. I'm ready to replace my Kelty Red Cloud 90, and the Sea to Summit e-Vent packliner I bought has been since replaced by cheap trash bags...
I don't know if I'm aggressive enough to start keeping a spreadsheet of the process, though. At the end of the day, seeing the first number might scare me enough to slow me down... and I don't wanna be frugal!