Hello there,

I wanna get a thick/fat sound like, let's say... Santana. Besides the PRS he uses and the amp settings, what can I do to create that fat sound? Will a compressor help?

For those about to rock, I salute you!
Screw around with the EQ. I use plenty of bass and mids and drop down the treble when I play clean because I like for it to sound more full and less twangy. A compressor just makes things tighter. I don't think it would help but I've never fooled with a compressor enough to know for sure.
My Gear:

Squier Affinity Stratocaster w/ SD Hot Rails (Bridge)
Yamaha F-310 Acoustic

Peavey Studio Pro 112
VOX AD50VT Valvetronix
Ross Fame Series Bass 50

BOSS DS-1 Distortion
BOSS DD-6 Digital Delay
like he said, pump the bass and mids and leave your treble pretty low.

bass 10
mid 8
treble 2-4 <--- theres my clean
"there is no spoone" - The Matricks
Ok. I definitely gotta mess up more with the Eq. Thanks!
For those about to rock, I salute you!
Hmm... You could also try rolling back your treble using your guitar's tone knob. The problem is, it's difficult to get warm, lively cleans out of a guitar with pickups made for thick distortion. You may even need to consider an EQ pedal to do what you want so you can really fine-tune your tone. This problem, of course, is compounded if you're using a brightly voiced or thinner sounding amp, which metal amps like the XXX, 5150, or Marshalls can be.
Hi, I'm Peter
Neck pickup, clean valve amp(like a Fender or something)
"Breathe, breathe in the air
Don't be afraid to care"

Fender Strat/Tokai LS80>few pedals>Orange Rocker 30
dial off some of the tone on your pickup to get a much smoother sound without the bite of all the treble/mid frequency's