Ok, a cool way to EQ follows thus:
When you are doing a mix (I'm assuming you are asking from this point of view), you might want to scoop out some mids from a guitar to make room for a vocal. Pick a frequency that you think might be on the money, say 2.5k, and cut that out. If it sounds good, fantastic. If it sounds wrong, bypass or switch out the EQ, adjust it to a frequency that seems like a better bet, and then put the EQ back in. Keep doing this until you get it on the money.
This might seem difficult to begin with, but pretty soon it teaches you what certain freqs actually sound like, and you'll then be able to listen to something and go "ok, that vocal needs a bit of 8k" and be spot on most of the time.
Generally this works better than sweeping. BTW this is not my idea, I learnt it off another guy (who I shall remain forever indebted to).