Keep your left forarm, as much as is possible, at a straight right angle to the back of the neck.
Keep your wrist as straight as possible. The greater the angle of your bent wrist, the greater the amount of resistance to tendon movement and likelihood of injury. Always keep your left forearm at as close to a right angle to the back of the neck as possible. This will require extra attention to the upper left arm and shoulder, particularly when playing way down the neck in the lowest positions. Depending on your arm length in relation to the neck length you may need to practice expanding outwards from your left shoulder to assist in the reach. Keeping the neck pointed upward at about 45 degrees gives you a more consistent reach along the entire neck. Most players tend to keep their left elbow tucked in close to their body and as they play further down the neck, causing the straight right angle of their forearm in relation to the back of the neck decreases while the amount of twisting and strain of the wrist increases.
Keep the palm of your left hand parallel to the bottom of the neck while aligning your fingers with the frets. You will notice as you reach further down the neck that the pinky side of your palm will tend to drift down away from the neck. Keeping the palm flat and parallel to the bottom of the neck is one of the more difficult contortions. Most players settle for the slightly more relaxed position where the thumb side of your palm is closer to the neck than your pinky side. This position causes your fingers to point more towards your face, making finger reaches down the neck on the lower strings increasingly difficult. It also weakens the 1st. and 2nd. fingers ability to play hammer notes. If you have some difficult lines to play in the lower register, you may need your 4th. finger to have access to the low "E" or "B" strings near the 4th. fret. To do this you will need the pinky side of your palm closer to the neck. Tip the bass up, bringing the 5th. fret closer to your face. This new angle of the neck should better match the natural angle of the flat of your palm and thus give you greater reach with the 4th. finger. "Ground 0" Position Place your palm flat on the bottom side of the neck in 7th position (a four fret position where your first finger plays in the 7th fret). This is a position where it is easy to maintain:
~ Your palm parallel to the bottom of the neck.
~ Your left forearm at a straight right angle to the back of the neck.
~ Your wrist absolutely straight.
A.) Point your fingertips straight out away from the neck as illustrated in illustration below. Be sure that you can see about 1" inch below the finger into the palm when sighting down across the fingerboard.
B) Bend your thumb back slightly so that only the flat soft surface of your thumb touches the neck.
C) Maintain the thumb position and palm / finger angle as you drop your hand down towards the floor, about 1/4" inch below the neck.
D) Maintain all the above positions as you curl all 4 fingertips around to meet the A string. Initially, it may be helpful to move all 4 fingers simultaneously and place them together in a row all on the same string. Don't worry about staying spread out over a 4 fret position. Allow the fingers to stay touching side by side.
E) Straighten your fingers out, back to "ground 0" position. Recheck your position for (A - C) above. Now curl them back again to place them all together on a different string. When in position with the finger tips on any given string, your first of three knuckle joints (where your finger meets the palm) should be in line with the 4 strings as you sight down across the finger board.