I usually wait until i find a total feckin bargain and then buy it, they usually come in the form of BINs on ebay that have literally just been listed and nobody else has spotted it yet or friends of mine happen to be selling them... Plus i just tend to be lucky and find the ones i've been looking for. Also basschat is an ace place for getting awesome gear at awesome prices, i wouldn't have two of my basses or my stack if it wasn't for BC
As for figuring out what i want, i dunno i go through phases of wanting different stuff, i've now got four basses, and out of them there's pretty much anything i'll ever need.
I tend to take risks to be honest though, i bought my musicman sight unseen and having only ever played one SUB bass before unplugged and one other stingray, both a good year + before i bought that one, i got lucky. Same with my jazz, i never really tried one properly, got this one in a trade, sight unseen again, and lucked out again!