If you start talking loudly, perhaps a little louder than you think you should be talking, attention will usually shift to you. If you say something relatively short and relatively interesting, you gain some credit, and then later people will be more likely to listen to you. This requires some timing - you have to start talking just as the previous speaker is finishing, or even a little before. Mistiming could make you look like (and be) a shouting idiot.

If two or more people use the same strategy (starting off loud), then you can use the opportunity to appear (and be) gracious by telling the other person to go ahead. This also subtly puts you in a position of control; they got to talk, but you decided who should talk.

(This is all based on personal experience, of course; YMMV.)