How about prevention? Anti-perspirants?
I've never used them, but some of them boast "no white marks". At least that's what they say on tv...and television is always right.

As for the clothes..maybe you should just wash them more regularly. I know my brother used to get those salt marks, but that was because he would leave them to stink and dry out in his room. When you get home from school/work, dump your clothes into a bucket of water (with some sort of cleanser if you want), so the sweat doesn't dry into white marks. Take them out and wring out the water an hour or so later.