The tool that seemingly does it all, EQ is commonly used, but often not very well. Maybe because it is used to compensate for something that came before it and that is never a good sign.
A useful lesson from EQ is that the very best use of any tool is to complement something. In normal use, you should only be applying minimal changes with it. If you have to resort to something drastic, then there is something wrong earlier in the chain and EQ is just another layer that can go wrong and not quite have that “radio-ready” sound you are probably looking for. Instead of thinking of it as useful treatment, think of it as a remedy to a problem and work backwards. Find the source and fix it there.
In the situations when you truly can’t do anything else, there’s another simple piece of logic that will do you worlds of good. When using EQ in any real-world situation, there are always going to be more frequencies that you want to keep than you want to get rid of. Hence this oldie but goodie: “cut, don’t boost” – meaning get rid of what you’re sure you don’t want, rather than amplifying what you think you like. Bad frequencies are likely something like footsteps, jewelry or keys, furniture, or other room noises. You can identify them easily and get rid of them easily. But good frequencies are much harder to understand. What is a good frequency? Even when you think you have found something awesome – like a particularly powerful sound in a vocal – you will be devastated when you find out the hard way that frequency problems are the main source of bad translation.
In essence: boosted frequencies will betray you, cut frequencies will no longer matter.