There are a variety of Outreach Tips that will aid in the successful execution of a public lecture. The complete list is found by clicking on the above link. For a Public Lecture, here are some specific issues to think about:
Be aware that, because of your popularity and the reputation of your lectures, you may get more people attending than you have seats for. This is where and when a project manager can assist you: Because while you are backstage getting your microphone set up, they can usher in the people and make sure people are sitting safely. If your lecture involves a demo that requires safety equipment (for example a fire extinguisher), obviously you need to have these things on hand as well.
Consider filming your lectures, making them a podcast, and posting them online – this will give you more promotional opportunities. Talk to your department IT or AV pro, or your university’s IT department which is probably happy to help you make podcasts and post them.