I've been around the block enough times to know what makes a good purchase - here are some of my best tips on buying microphones, but most of the information here can be applied to purchasing anything in general.
There should be some thought before you buy mics, and research done. Be
careful when you look reviews up on the internet, especially when
peeking at low-end mics, because people are usually really excited and
possibly even just getting their first mic that isn't from radio shack.
So basically, don't believe everything you read. Example: "d00d thiz mic
is my first and last mic. it rawks my sawks with itz ultra-high end
shockmount. behringer4life sw33t".
Ignore no-name brand mics. No-name brands don't make anything in the
high end, so if you are considering a Behringer or Samson mic, you are
not looking to buy a great mic. So ignore these brands and get something
from the lower end by a reputable company. If you grow out of your mic
purchase, it's much easier to sell a shure PG57 than it is to sell an
Optimus rx90 turbosound w/ on off switch. [I made that last one up.
Well, the brand is real.]
#3 buy in stereo if you can.
#4 Consider what you are buying before you buy. Fewer decent mics make
for a much better recording than more shitty ones. For example 10 crappy
mics you get for $399 isn't gonna make your drums sound great. If you
buy 2 or 3 decent mics for that price, chances improve.
#5 make sure your mics are realistically multi-purpose. I can't imagine
you are going to use a beta 91 for more than a kickdrum, so you'd be
better off with a beta 52. Instead of an audix d2, try a sm57 or 56.