I pack me check-in bag with firm expectations that there will be a rummager's hand in the bag at some point, from experience. Dirty clothes (on the return trip) around the outside, anything I'd really rather not lose but can't take in hand baggage (e.g. expensive lotions / perfumes over 100ml) underneath something hefty such as a pile of books or some shoes or wedged in deeply at the back, designer shirts underneath the cheap T-shirts. Basically make the bag contents seem as unattractive a target as possible at the first opening of the zip whilst potentially concealing some more valuable rather-not-be-stolen stuff. With 1000 other attractive bags to steal from who is going to spend time rummaging through my laundry? A lock helps of course.