I think a good yoke pad is a big thing. Use a good one and you'll never settle for a thin hard foam pad again. Although some people swear by the countered all wood yokes that are cut to distribute weight across your back and shoulders.

As for exercises, Overhead presses with dumbbells are good, but just simple shoulder shrugs with the same are great too.

Just keep dumbbells close to sides of body and " turtle" up and then shrug your shoulders back and lower. Start light, maybe 10# or so and do a few sets of 8-12, every other day. Work you way up to say 20-25lbs.

Unless you doing a lot of serious overhead/neck work like a football player, you'll find these smaller muscles in "normal'" people fatigue quicker than you might think.

Better yet IF your with a partner and he insists on the thin pads, you carry the  heavy pack and let him carry the canoe ;)