[quote name='debwilliams' timestamp='1345585769' post='1779248']
I've begun to notice that when I'm playing things that are more challenging I'm holding tension in my jaw.[/quote]
Is this not the oft-encountered [i]bass face[/i]?
I pout my lips like an 80's glam rock star when I get going. The only saving grace is that the drummer [i]gurns[/i], which draws the eye from my own efforts.
Try some basic relaxation techniques, of course; however, I think the trick will be practice, practice and practice. I only ever really seem to relax when I know my stuff. Once you nail it, try challenging yourself - playing it whilst moving, etc. The confidence and, in large part, competence and muscle memory, I find allow me to unlock other parts of my performance.
When I started, I couldn't move and play - now you can't keep me still; I couldn't sing and play - now you can't shut me up; etc.