good shoes with arch support is the key to avoid back pain, we arent young anymore so flip flops and bare feet are fine around the house but on the boat, not so good.
those flexible back support braces (just elastic bands really) they do help a lot and they tend to give you better posture as well. something like this:
http://www.walmart.com/c/kp/back-support-belts just avoid the fancy low profile stuff or wonder magnets and just get a good quality one to support your back.
I have used the cheaper ones and they work ok but you do get what you pay for and I use this one and its well worth the extra money
http://www.walmart.com/ip/MAXAR-Work...pport/20746309
I use them when fishing or on the lawn mower, anytime im getting vibrated or bounced around or working in the yard doing a lot of bending and lifting.
it is a little bother in the heat of summer but your back will thank you.
another very overlooked item is a soft shock absorbing comfortable seat to absorb bouncing if you sit to drive or sit at the trolling motor all day. a good seat goes a long way to literally not being a pain in the back side