As with anything, the following are some helpful guidelines for EQ. When EQ'ing, it's always best to cut instead of boost, but boosting is necessary sometimes. Boosting specific frequencies changes the sound, cutting just helps clarify and define (until you cut drastically).
Here is a general guideline I have derived from different sources. It's a living document and can be changed/added to at any time to help define its effectiveness. If anyone sees any problems with this chart, let me know and I will take that into consideration for updates.
Above all, if something in this chart doesn't work for you, trust your ears! This is a general guideline and might not work for the type of music you're working on. Don't try to do ALL of these things, either, only adjust EQ when you NEED to. EQ will NOT fix a bad sound, but it will enhance one that's already decent.
Remember that when you increase a frequency, what you are doing is increasing the volume, or gain, of a band of frequencies centered around that frequency and with gain comes boost of everything you've recorded in that band, like noise. That's one big reason to cut first, boost later!
Good luck, everyone!