I have heard so many things about where your thumb is supposed to be placed when you play, but all of these seem really awkward for me. They say that you are supposed to imagine a halfway line running along the back of the neck and never put your thumb higher than that. With my abnormally large thumb, this is very uncomfortable. I understand that new things may feel weird at first, but doing this makes me end up with a really flat, sloppy hand. And when i get down to the lower frets, my thumb wants to just wrap all the way around the neck. Any advice?
personally, i wouldnt worry too much about it. i think its natural for the thumb to drift away from that halfway line, especially when playing the higher strings.
Good hand posture is always a good thing to strive for but it's okay to relax your hand when you need to. Whatever works best for you is usually what's going to be the route to take. Although instructors will always stress the importance of a 'correct' posture with the fretting hand. It works for some and for some it doesn't. I think more advanced players demonstrate correct left hand positioning and posture fairly evidently which would suggest that those who preach it know what they are talking about. But it is a personal thing...if it works for you then do it. If it doesn't then don't.
Ok. I guess I am looking for too much advice when i should be letting it come more naturally. Thats why i am afraid to take lessons. A teacher would just take me back to the basics and tell me all the things i'm doing wrong (or at least what THEY think is wrong).
I think the reason instructors stress posture is that they know that in the long run it pays off. regarding thumb on the left (fretting hand) I was instructed to position is behind the neck approximately in between the index and ring finger. This is difficult when I play high on the neck so I let it slide a little in those cases.
Havic5 has great advice on left hand technique, including thumb placement. Developing Safe Left Hand Technique for Bass Guitar - YouTube
I find the Simandl method, used by upright players, is best for facilitating left hand shifts while emphasising minimum movement. Check out any of the following: New Method for the Double Bass, Franz Simandl Ray Brown's Bass Method Building a Jazz Bass Line, Ron Carter Modern Walking Bass Technique, Mike Richmond