Another good rule of thumb when mixing is to first remove EQ frenquencies you don't need first before adding more. The more you can remove the less cluttered the mix will be.
Playing with a parametric EQ on a track by track basis can yield awesome results if you take the time to figure out what is causing it to get cluttered. Perhaps for example your lead track is sharing alot of the same bandwidth area as your rhythm tracks. By adjusting the EQ on the lead track i.e. moving it over a little bit to the right it will pop out more without raising the level because previously its competing for space with the rhythm guitars.
You have to understand that EQ is for shaping if you start adding too much EQ your turning the EQ into a gain box which its not meant to do and your going to create alot of unwanted noise. EQ should be used in as much moderation as can be.
Lets assume for sake of arguement that your rhythm tracks main meat and potatoes is at 450 to 550 then you realize that your lead channel is also pinging that frequency. if you take your lead track and move the meat of it to bottom out at 590 for example you just increased its perceived volume in the mix without adding any gain to the channel at all.
Andrew is also right too adjusting your stereo field makes a huge difference too, when multi-tracking guitars i usually have it panned left and right about 80% but for my lead i dont leave it straight up and 0%, i usually will pan the lead left or right about 10 to 11% so its not competing for virtual realestate with the drums which usually are right around the dead center mark.
A good plugin that i use to see frequency spectrum is Voxengo Span i put it on the master as the last thing in the effects chain and it allows me to see the spectrum frequencies of what ever track i am soloing or to see the mix as a whole.
this allows me to make EQ adjustments to specific tracks with a knowledge of where the meat of that frequency is.
You can get SPAN here its a free VST plugin
http://www.voxengo.com/product/SPAN/Daniel