I have a warwick 5 string german bubinga. Its pretty sick.

It also weighs more than a small child, and my last few gigs my left shoulder has been bugging me.

Any suggestions, any good straps that might help?
I'd say the wider the strap, the better. But the most important thing is to play while standing as often as possible - that means not only the when playing gigs, but also when practicing at home. The pain will go away after some time.
I have a JHS Vintage V8004B, also bubinga, which is pretty heavy to play standing. You just get used to it after a while.
You could look into duo straps or double straps.. I mean, they would take of course time to get used to, but they will even the weight either on both of your shoulders.
Gear pics

Quote by Cathbard
Bugera cloning Blackstar is a scandal cloaked in a tragedy making love to a nightmare.

Wide strap for certain. And as someone with back issues, work on your core muscles. The chances are you're getting tired because your shoulders are doing all the work.
1. Definitely get a wide strap made from a soft material. I have one of these and it's ****ing heavenly.

2. Improve your posture. Play in front of a mirror and chech your poise and technique like you're weight lifting or something. I found that I was slouching my upper back and shoulders which was causing my back pain.

3. If your bass is below your waist then grow up. The most confortable position to play (for your back) is to have the strap short enough that the bass doesn't change height when you sit down. (However this is uncomfortable for the arms and wrists of some people, especially if you have long arms.)

4. Just a minor point, shift the strap so that it isn't pressing on your trapezius muscle.