The earbud clicker has two main functions: answering/hanging up a phone call, and play/pause/fast-forward music. Unfortunately, the latter doesn't work if the iPhone has gone to sleep. It's already happened to me several times that I'll pause my music to talk to someone and then discover that I can't resume my music by clicking the earbud. The simple solution: tap the power button on the iPhone, then use the earbud clicker. Your iPhone will stay in a locked mode, but your music controls start working again.

This trick isn't as good as the period-typing shortcut (via), but it's been convenient for me. There's also the apostrophe shortcut for we're, he'll, and we'll.