The stretch/shift boundary of fretting hand technique.

[air_leech]

Originally posted by Son of Spam
I don't know about the rest of you, but my position seems to switch rapidly.

whatever makes her happy mate

[fat-bottom]

The "finger per fret" thing is great in theory and if you play a lot of chromatic lines.
If you play tough stuff, you know it's not all that easy. You have to position your hand to play the line as effortlessly as possible while thinking ahead to your next phrase.
Complex lines require a good deal of hand movement and dexterity. I suggest using stretching exercises, positioning your bass so that your left hand is under the neck and moving your left elbow as close as you can to the left side of your chest. This gives me the ability to play anything I can hear, providing I can play what I hear!!!!

[Thunderpulse]

Maintain a nice relaxed cup position with the fretting hand.

Never press your thumb into the neck.

Nearly all bassists' thumbs peek over the top.

[Pilgrim]

Learn the basics first, including proper thumb position and finger fretting.

When you are as good as McCartney, feel free to depart from those practices and invent your own style.