As much as I personally hate the idea myself, it truly is first and foremost about getting a frequency balanced mix first before you dive into getting the leads to sound right, which is the idea Crucified was pointing at.
Too much high mids (which is generally going to be the killer here) in the rhythm guitars is something that will totally destroy the ability of the leads to cut through (not to mention it also robs the vocals of breathing space).
As much as you're a guitarist and feel tempted to want to boost the shit out of both the rhythm and leads, it just can't work that way.
Boost both, and you make a mess of things.
You simply gotta sacrifice a bit of that rhythm guitar sound for the leads, but it's for the betterment of the whole mix anyway, so it wont sound worse.
Don't be afraid to compress, limit and use saturation if you have.
Compression/limiting on leads can help stop those occasional notes that jump out annoyingly at you, and saturation can make it a little more mid focused (not in the same way as EQ of course, but it works).
Multiband compression on rhythm guitars is another good one, for helping to give space to a mix for all the other elements.