One of the easiest ways to alter your guitar sound is to experiment with different sizes and gauges of picks. Using a very thin pick produces a sound drastically different than using a heavy one. Many jazz guitarists tend to favor using heavy picks (1.5 or 2 millimeters) because it tends to thicken and darken up their sound somewhat. I personally use a 1.0 mm pick as I am a jazz fusion/rock guitar player.
Trying different string gauges can also help when searching for that perfect tone. If you are a jazz or R&B style player, a heavier gauge may be the ticket. If your style of music requires you to do a lot a string bending, then the lighter gauges are probably best (thin strings keep the blood to a minimum).
