There are many factors that come into play which defines a guitarist's sound: the amp, effects, guitar, strings, pickups and so on. However, one often overlooked component is the pick! The thickness, size and material it's made from all have a dramatic effect on your overall sound.
As I'm sure you all know, picks come in a variety of sizes from large triangular ones (like the type used by Carlos Santana) to very small, mandolin style picks. Common materials include: plastic, nylon, tortex (a man made substance to approximate tortoise shell), and even metal and stone picks are available.
The thicker the pick, the more volume, and to a point, the better the tone. Some guitarists may disagree with me, but I have found the light flimsy picks simply don't sound very good. Much of the dynamics that can be coaxed out of a guitar are just much easier with a thicker pick -- at least of medium thickness. Too thick is not so good either as some can make you feel you're playing with a cough drop! As far as a preference to pick material, everyone has their own opinion. To me, most plastic or nylon picks sound too clicky and brittle, whereas tortex picks seem to have a darker tone, with almost no click but still has the same presence you get from plastic, nylon or metal.
Size matters a great deal as well (no double meaning implied). I find larger picks are too hard to handle, whereas smaller picks are easier to control. Also it's much easier to palm the pick when bare fingers are required. The pick I use is a Jim Dunlop Tortex Jazz III: Heavy. It has a very sharp point which I find easier to squeeze artificial harmonics out with, and being very small I find it easier for hybrid-picking (pick and fingers) as well as less material to deal with.
If you admire a certain guitarist and wonder how he or she gets that sound, don't stop with just a particular guitar or amp -- find out what sort of pick they use. Billy Gibbons from ZZ Top uses a Mexican Peso, Roy Buchanan used a penny, and Brian May of Queen uses a schilling (speaking of metal). Ritchie Blackmore, formerly of Deep Purple, makes his own diamond-shaped picks from heavy plastics, while Eric Johnson uses Jim Dunlop Jazz III red nylon picks.
Experiment. I'm sure you'll find how a certain pick can truly help define your own personal sound!
Greigg Fraser is a guitarist and recording artist from Ontario, Canada who produces totally original instrumental compositions melding both clean and tortured, distorted rock guitar with fusion, new age and pop overtones.
His latest CD is entitled "Under The Radar".