The best way is to have another meeting you have to go to. Just like on a first date where you have a friend call in after an hour with an “emergency”.
 
But you have to do it just right— the meeting you are leaving for has to be with someone much more important than the one you are attempting to leave. That way they can’t pull rank! Of course your coworker needs to stick to their story. If you say one thing, and they say another later, then the jig is up.

There is also the Fake-A-Call app on the iPhone. Results have been mixed.

additional comments from chat:
Viewer103: Dysentary
smarba: emergency call from your spouse. Works every time.
Pedro: Wife is having a baby.
Rick-UK: left the dog in the oven.
Billy: Claim you have explosive diarrhea.
czmilosz: I got out of a meeting last week by scheduling a vasectomy.
MeetingBoy: I suppose the bathroom is good, but you have to really sell it. I would give everyone food poisoning to really pull it off.
Paxochka: So if you’re catering the meeting, MeetingBoy, remind me not to eat anything.

MrFunkyStuff: I think lying is morally wrong. Any other suggestions?
MeetingBoy: Oh, sure, MrFunkyStuff, as though lying isn’t part and parcel of office work.

This was a question from the @joinme interview. Read the rest of the MeetingBoy interviews.

join.me, the last two words in an invitation to collaborate, meet, train, demo or show-off. Try it now at join.me.