Anytime I do an open mic is an exercise in public speaking. But public speaking isn't limited to standing in front of a crowd at a specific event. Think of all of the times you've had to do an oral exam, or had a group meeting at work that you were asked to contribute two cents.
Next time you go to an event with a Q and A, ask plenty of questions that are clear and to the point. If you're at a gathering, see how many conversations you can follow and contribute to.