Here is advice I got from dlloyd on my technique:
The "correct" playing position is with the body of the bass in the position it would be in if you were sitting with it on your right leg. So you should adjust your strap to suit this.
The headstock of the bass should be raised to about the level of your left shoulder. So there should be somewhat of an upward angle to the neck of the bass. You should find that, with your thumb at the back of the neck in the centre in the position I described, your wrist will naturally be straight.
The left shoulder and arm should be relaxed and the thumb should only be used as a guide/rest for the hand, with no pressure exerted on it. One way to achieve this is to practice playing with the thumb hovering over its usual position (and avoiding touching the neck with any part of your hand that doesn't usually touch it )
It's possible to get the neck into the correct position for fretting in lower positions by wearing the bass higher with the neck horizontal, but you're then in trouble with your right hand.
You see a lot of rock musicians wearing the bass a lot lower than this. I agree, it suits the rock image better. What you'll tend to notice is that a lot of them will have their hand around the neck so that their thumb is around the top of the neck, leading to a straight wrist.
You do see a lot of bassists with bad technique. It's a natural by-product of the self-taught school of thought. What's worse than this is that a lot of guitarists and bassists who are self-taught (or badly taught) go on to become teachers in their own right and it never occurs to them that they are passing on bad habits that may be damaging.
The damage you experience with tendonitis and carpal tunnel syndrome is a cumulative effect. You may go for years thinking your playing is comfortable and everything is fine, until one day you realise that a couple of your fingers are numb and lack any strength or that fretting gives you searing pain in your wrist or hand.
I went for years using bad technique myself and wished I'd been taught correctly in the first place. It's much harder to unlearn bad technique than it is to learn it properly the first time around.
Definately try that out, works like a charm.
