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 over looked 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 - 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, where as 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, where as 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 - less material to deal with.
If you admire a certain guitarist and wonder how he or she get's
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 II 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/songwriter
from London Canada. Click below to visit his web site and listen to audio samples from his two CD's!
http://www.clevernet.net/curlymaple/