Put your bag in upright rather than lay it flat, and then no one can put anything on top (and the added advantage is it gives others more space to put their things).

Occasionally my rollaboard won't go in to an overhead on its narrow side (depends on the aircraft). If it has to go in 'flat' (so people could put stuff on top) I will put my coat / purse on top on my own bag to prevent someone squashing my delicate items. Of course if it is just clothes or something, that's different, but there are normally at least a couple of breakable items in my rollaboard (duty free, for example).