It happend to me a couple of times now that I've cross-loaded my carabiner when belaying; the frequent change from slack to tension and back, moving the carabiner up and down and all that, can easily result in such a situation.
There are a few biners adressing this issue by adding a small wire gate lock at the bottom or having a 8-like shape, but I was wondering whether anyone knows a neat trick how to keep it in position so that it won't turn.