Simple: You don't.
In two different instances this past week I've found people are more interested in stimulating a conversation about the operating system if you let them ask the questions - don't force information on someone they very well might not want or even care about.
I'm currently taking a Number Theory class this trimester, the class involves a lot of algebra at times. So I don't get bogged down with such menial work I typically always have my netbook out with wxMaxima. As I was going through some group work with a few other students this past week, I was showing the results of some equations I had the computer solve to a classmate when he made me pause for a moment. The conversation went something like this:
"What was that?"
"What? You want me to double check the numbers?"
"No, not the math. Your window, when you moved it - what did it do?"
He was referring to the "wiggle" my window had when I moved it around. Compiz is a wonderful eye catcher. I then proceeded to show him the desktop cube and a few other effects that are easily enabled. After a couple of moments he asked my favorite question:
"What version of Windows is that? Its pretty neat."
I mentioned that it was not Windows, but something else - Ubuntu. Alright, I lied before then he asked my favorite question:
"Oh, never heard of that before. How much did that cost?"
He then asked:
"Free? Really? What can it do?"
"Oh, you know everything you expect a computer to do. Type a paper, surf the internet, solve math equations, play games..."
He then asked where he could get it from, I gave him the web address and told him if he had any questions about it he could feel free to ask me next week at class.
This is the right way to get someone interested in something. You don't force it down their throat or rant and rave about how it is better than something they are already using. People are often comfortable with things they find familiar and they are naturally resistant to change. Many users (myself included) find themselves over excited when they first discover the world of choices that open source presents them. As such I think they feel it is their "duty" to make others "see the light" and "convert" to this new way. Even though they do not mean to - this attitude hurts the image of the FOSS/Linux community and is where the negative term "Linux Zealot" comes from.
In closing, if you feel you must make the general public aware of Linux remember this: mention - don't preach.