Or do you buy one that far exceeds what your hardware requires so it can last 2-3 upgrades?

I plan on upgrading every 2-3 years.

I used to upgrade with every build when I built with cheap, crap PSU's. Now I have one that exceeds my current requirements, that why I can SLI should I want to, or upgrade and still have a nice PSU. I've come to prefer flexibility over bare-minimum.

I try out the old PSU first on an upgrade. If it works I stick with it.

I upgrade about every 2-3 years, and when I say upgrade... I mean a totally new system. the old one goes to family members or file server.

Ok back on topic. I usually upgrade/replace the PSU every other build because of capacitors get worn out over time.

In 2000 I bought a bunch of 385/430 enermax PSU's (which were big for their time) then replaced them with 700 OCZ GameXStreams a couple years back. So no, I do not replace them all too often despite my many upgrades, but then again I don't buy the cheapest PSU out there either. The Enermaxs did die though, which expediated my need for replacements. I would say 4 years is probably a good expectation for a PSU under heavy usage.

Thats a bit of dangerous thinking there... i hope you at least look at the specs of the psu and the system your putting it into first, you could damage other components with this method.

Generally when i get a new machine i buy a whole new machine and my old one travels down the line (ie sister, mum , dad) so its usually a new one each time, but if you buy a good solid PSU and you upgrade, no reason why you cannot keep it for 1-3yrs if its still alive and able to sustain stable power to the upgraded systems.