PREPARATION:  GUITARS

Amps/Cabs:
In general, tube amps sound best.  Make sure your power and preamp tubes aren't too old.  New tubes will have more gain and better high-end than old tubes.

For high-gain guitar amps, I recommend Peavey 5150 / 6505, Marshall JCM Series or Mesa Single / Dual / Triple Rectifiers.

Make sure there are no blown speakers on your cab and that you're using a speaker cable to connect your amp head, NOT an instrument cable.  Also, check and make sure the ohm rating on your head matches the ohm rating on your cab.

Guitars:
Change your strings the day before or the day of recording.  This will ensure the best possible sound coming from your guitar.  String gauges should be appropriate for the tuning you play in.  Bring an extra set or two in case of breaks.  If your guitar has active pickups, be sure to change the battery.  Though usually less apparent than with bass guitar, intonation can still be an issue, so make sure your guitar is set up by a professional before recording.  Bring the shortest length, highest quality cables you have.  Instrument cables are unbalanced, meaning they are susceptible to interference.  The shorter the cable, the less interference.

Performance:
Practice, practice, practice!  It should go without saying, but make sure that all guitar players are clear on what they should be playing.  For punk, metal and hardcore, pay special attention to palm muting.  Nothing sounds sloppier than having a doubled rhythm guitar part with inconsistent palm mutes.