Some good advice has been offered so far and I believe I may be able to add to it. While I'm not a member of ARMA, and while my material may be different than yours, I think some basic principles still apply.
This last weekend we attended a small renfaire in Wichita, KS. Here's a photo of what I was wearing.
Considering this was a renfaire (with the usual stereotypes) and not a living history event I think everyone can assume that I was the most 'historical' one there. As can be seen, my main interest lies with the equipment of the Normans during the 11th century.
Over the last year my wife and I have been attending events of this type with friends and I have also given quite a few presentations for organizations such as the Boy Scouts, local highschools, elementary schools, etc. At this particular event I was approached about giving a presentation at a larger non-renfaire venue in downtown Wichita later this year.
These experiences have proven a few things to me: The average person isn't as stupid as we'd like to believe. Even if they don't know anything about the subject they can tell the difference between a quality presentation and one that's been thrown together and displayed with ignorance. In fact, the general public seems to be getting a bit more sophisticated in what they want. The standard SCA demo that we've all seen at renfaires for decades really doesn't seem to hold their interest anymore. When your presentation is more historically focused and more "real world" rather than "let's pretend" they eat it up. In this sense this is the perfect time to get out there and spread the word as the public seems to be ready for it.
One thing to remember is that you need to approach your audience with an assumption of ignorance. What I mean by that is assume they know nothing about the subject. This means avoiding the use of complicated and, to them, obscure terminology. Explain what you're doing and why in plain english. Also remember that you're only hitting the high points. You have a limited time to communicate so don't try to jam everything in. Approach it with the mindset that your mission is to generate interest. Your objective is to make them want to go find out more on their own, not to turn them into scholars of arms. Give them the tools and point them in the right direction. One very helpfull thing is to write out an outline of your presentation. This will help you be more organized and fluid. It will also help you avoid being sidetracked by any questions that pop up.
Another point, and probably the most important one, is to check your own attitude. Nobody's interested in listening to a know-it-all criticize other methods or groups. "We're better because...." won't win you any points with the audience. People don't respond well to members of the "look at me" club. You may very well have people in the audience who are self-proclaimed experts. Don't feel you have to get into a contest to prove your point, simply bypass them and move on. You won't change their mind, because they're just there to argue not to learn. Checking your ego cancels out theirs. Simply let your presentation and the information speak for itself.
There are many people out there who share your interest but who feel intimidated. Your gear may be better and you may seem to know more and they'd like to approach you but they won't, probably because they've had contact with arrogant jerks from the "look at me" club in the past. This happens to me at every event I attend, including this last weekend. On several occasions I was being followed by people who were obviously interested in my equipment but wouldn't approach. Fortunately my very perceptive wife picked up on this on eventually waved them over. I don't care if my stuff's better than yours, we all started somewhere. I also love meeting people who know more than me because I learn something from it.
Rather than approach the subject from the point of "Our job is to correct the mistakes of those other dumbies.", go into it from the standpoint of "This is so cool and exciting that I want to share it with you." Attitude is everything.
