While we're on the topic of messenger bags, how do you guys keep the bags from sliding around to the front of you while you're riding?

I've been having this problem with both messengers and one strap sling over backpacks. When I ride occasionally the bag will slide over sometimes far enough to be banging against my leg until I sling it back over. Is there a way to keep this from happening? Or is it just something that happens with the type of bag that I'm using?