I guess starting with buying, if it's a mic I've never tried before I will always audition before purchase, unless it's quite cheap, say $300 or so. Exceptions of course if I have excellent reports from people I trust, and know their sense of aesthetic. There are very few of those people. So it's a demo, hopefully for at least a week if not a bit more to see if it will fill a hole in the arsenal.
For using it depends on the situation. If it's cowbell, I typically just grab whatever mic happens to be on a stand close by
. For anything a bit more critical, but still not of utmost importance, I pretty much have a set of candidates that, depending on how I want the source to sound in the mix, combined with what the source sounds like, I can usually narrow to a mic I think will work best, or maybe 2. This just comes from experience and being familiar with my mic locker. I would say I get it right first try 80% of the time, but I still like to try new things on a semi regular basis, just to make sure there's not something I'm missing that would work really well. For example on snare drum I've got 5-6 mics that I know quite well what they'll give me. However as was mentioned in another thread, Matt had picked up a Beyer M201 that had been living on electric guitar since he got it, but we knew we'd have to try it on snare sometime. So when an appropriate session came up we used it for the whole session with a few different drums. I can't say I'll use it for every drum from now on, but it's certainly a favourite, and it will get put there again sometime in the future.
For anything super critical, or if I'm recording an instrument I've never heard before, I'll do a quick shootout. Typically no more than 4 mics because it's hard to keep track of more than that.(Also I only have 4 really good matched pres) I'll put up my best guess', record them all on the same take with them as physically close as I can make them, and pick a winner. If there's still nothing that's really doing it for me, I'll pick the least bad option and throw up 3 more mics. I almost never have to do that.
All that being said, at least half the time, as long as you're in the ball park(ie not putting a ribbon mic 3 inches from Matt's snare drum) the mic is not all that important. If you're working with quality gear in a decent room you should be able to get good results.