1. Sit down. Yes, this helps, because it would mute the loud sound, if not completely, then substantially, and chances are that people won't hear you letting it pass.
2. Tighten your rear as much as you can. This should be done as if you don't want to let it pass. Even though that won't be possible, it would make the action more silent and you, less embarrassed.
3. Look as straight-faced as you can. If you get conscious, people would know the source of the smell. In fact, try to hold it while you are speaking, because your uninterrupted speech would be a proof that you didn't do it (even though you did). more