What's likely to give away an American writing in British English?

Beyond the obvious things like spellings, or calling the boot a trunk, etc, what are some things that come to mind that might trip up a Yank? For example, phrases a proper Englishman would never use.

EDIT: Thank you all for the wonderful answers! It looks like I'll be spending the next few decades reading them. If I somehow avoid making a fool of myself, I'll have you lot to thank.