A “Clean Sound” is the most important tool.  It is comprised of several key ingredients the first of which is proper tone.  Using proper pressure on your fret hand fingertips and striking the string with the pick or finger (or fingers) to produce a cleaning ring.  The second ingredient in the mix is muting.  A little ironic that a clean sound requires no sound, until you realize the other strings need to be kept quiet.  The final ingredient is in the pick control, bass players who choose to use fingers are not exempt from this part.  Proper control here is essential.

Muting can be achieved with various methods.  The pick hand, side of palm and little finger, can be placed on the picking side of and at the bridge.  This form is controlled by pressure on the string, the amount of palm touching the strings and how far from the bridge toward the neck you are applying it.  This is a very effective technique to use if you are purposely playing muted notes. Using free fingers to touch strings not in play is effective to control any pick overstrike on a neighboring string.

Other Muting techniques use the fingers on the fret hand.  While you press on one string to play you touch the other string beside it with the side, the pad or the knuckle to silence it.  You can use free fingers (or the thumb on string 6) to touch a string to silence it and you can slightly relax finger pressure on the fret just played.

Pick Control begins at the grip, I suggest a thick pick for beginners.  Do not hold it overly tight, this will result in thumb fatigue.  The pick should be loose, but not beyond any lose of control.  Place the pick between your thumb pad and first finger.  The thumb should be placed in the middle of the pick and the pick resting upon the tip of the first finger.  You should be able let the pick slide along the top of the finger.  By sliding the pick up to the first knuckle you increase attack control for fast solid sounds, while sliding the pick back to the fingertip you relax control for slow meaty sounds

The Down Stroke is a very basic stroke, simply place the pick above the string(s) to be played and push the pick down across.

The Up Stroke is the opposite in every respect, you place the pick below the string(s) to be played and pull up across.

The Check Mark is a quick down then up stroke with a flare, you give the strings a little extra pull on the upstroke.  Pull just enough to accent the note.

The Circle is good for a quick repeating a single note.  You place the pick above the string to start, then, using your pick like you would a pencil, you draw circles around the string, striking across it on the down strokes and the upstrokes.

Vibrato is kind of fun sometimes, you place the pick on the string and basically spasm your hand from the wrist down striking the string every which way.

Finger Picking increases your range of string combinations you can play, by varying the amount of finger pad or tip you use to grab the string changes sound, volume and twang. Plucking 2 or 3 strings to play a chord will give you a range to and above 1 octave in tones.

The Thumb Slap is primarily a bass technique in which you pluck a string with your finger, in one swinging motion you quickly dropping the heel of your hand while extending your thumb toward the fret board.  You then strike or snap the top of the string with the knuckle on the side of the extended thumb while quickly lifting the palm back away from the strings.