Ah! The bane of my existence!
Seriously, though, how to avoid? How to get rid of? I have tried numerous kinds of deodorants, and I have come to the conclusion that I will always have marks on my dark clothing. I have pretty sensitive skin, and have found that Dove (the kind that isn't supposed to leave marks) is the gentlest. But, alas, it leaves marks.
What to do? I need a real deodorant because I bike/walk commute and I sweat. But it must be gentle.