How do you break off a conversation?

When you are 'working the room' at a conference or banquet - what is the best way to break off a conversation with someone? I have used "I'm going to go grab another drink" - but it doesn't always work (like when you have a full drink in your hand.) What is the best way to break off a short conversation - 'excuse me, I have to go' (and then run away like you have diarrea?)