Upside down?

I've been looking through lots of great bento photos at flckr lately... Many people use little cups or containers to divide different foods and also for sauces - and many times without a lid. How do you keep your bento box "safe" on the way to work or school? Even if I pack it very carefully mine seems to be more often upside down than not when I reopen my bag to put in the fridge! So I'm curious how often you find that your beautifully packed bento has turned into a big mess? (hopefully not often...!)