Do you like to hear lead guitar parts on an acoustic guitar that are kind ambiguous--that is, where the pick dances lightly over the strings, and it is not really clear exactly what is going on? Or, do like to hear a lead break that is snappy, where the notes are bold and crisp and make a clear statement? I definitely prefer the latter.If you are a guitarist who would like to go the "bold and crisp" route, let's start with the basics I always teach. The first thing you need to do is to start with a pick that is thick enough. Use at least a medium pick--usually about .73-.81mm. This may seem difficult at first, but it is absolutely necessary, so that you'll get strong, snappy notes.
Grip the pick firmly so that your thumb and index finger cover most of the area of the pick. I use a standard Fender 351 medium pick (or something comparable). Instead of holding it lengthwise, I hold the pick so that the top (that is, the shortest side) of the pick is lined up over the top of my thumbnail, and so that the edge of the top of the pick is hitting the strings. This may or may not be best for you, but, in any case, choke up on that pick to keep those notes loud and crisp.
Do not rest the palm or wrist of your picking hand on the bridge or on the top of your guitar. Your hand needs to float freely so that you can keep the edge of the pick at pretty much a 90 degree angle from the guitar top. Keep your pinky or ring finger (or both) stiff so that one of these fingers glides loosley over the top as a reference. This will definitely seem awkward if you are not used to it, but it is a must if you're going to be a good acoustic guitar picker!
Now that you have the fundamentals in order, think pendulum. A pendulum, by dictionary definition, is "a body so suspended from a fixed point as to move to and fro by the action of gravity and acquired momentum." Envision your picking hand as a pendulum, swinging freely from your elbow, with the pick held at a fixed 90 degree angle, dipping down to strike a string from the left--and then the right--and then the left--and then the right. The motion is not from the wrist, but is from the pendulum swinging from the elbow.
Now, simply implement what you have envisioned. Make that picking arm work like a pendulum, with the pick swinging down to cut into the strings. Here are some exercises to get you started. Play the low E string with 8 strokes: down-up-down-up-down-up-down-up. Now do the same thing on the open A string. Now, without breaking the rhythm, play the 8 notes on the E string and then the A, then the E and then the A. Now try the same thing but with only 4 notes each string: down-up-down-up. Now again, with just two notes per string: down-up. As you do these exercises, apply the pendulum principle. You'll find that your notes are beginning to sound like the notes of a real flatpicker!
