wires got some great suggestions there. basicly a distorted guitar and many of the other sources will share alot of these mid freqs and often they can muddy up or phase each other out. when recording the guitar make sure to listen for problems in cymbal and guitar relations. alot of times a crash hit can wipe out a good sounding guitar. all those upper mids just phase out. so as above... pan, double. think of specific sounds for specific parts. give your lead part a brighter sound which will cut through that middy mush. give rythem guitars a little bottom so you feel them underneath the mix (hopefully blending with the bass). with a little playing around with the eqs and pans you should be able to pull the guiter into its own slot in the mix.
you think skas a pain for guitar, try some death metal. all the cymbals, clicky blasts, clicky middy bass, layers of mesa triple rec amps (which to this day i cant get a good recording of. cant stand em in the studio). it can be a damn phase nightmare.