Hi There Big Bud. I'm happy to offer a few suggestions to minimize your back strain. I am a gigging bass player on weekends and a practicing doctor of chiropractic during the week, so this is right up my alley.
First, diet and loose any tummy fat (if you have some).
Second, strengthen all of your abdominal muscles with daily exercises.
Third, stretch all over before a gig.
Fourth, if you stand "swayback" while playing, tuck your pelvis to decrease the curve of your low back.
Fifth, wear a low back support belt while loading and unloading your gear.
Sixth, stand with your weight evenly distributed on both feet, or move in an overall balanced manner.
Seventh, wear flat shoes with thickly padded soles.
Eighth, stretch again between sets if you feel some back strain coming on.
Ninth, go back and sit on your bass cabinet during the ballads (if you're not singing)....I'm assuming here that it's a 1-15" or a 4-10".
Tenth, get acupuncture, chiropractic, and pro massage weekly to heal the tissues and joints of your back.
Remember Big Bud, you are no good for anything if you wear out your back....so take care of it now before you end up with a bulging disc and surgery.

Tisabelle