Here's my formula:
1. Do not do so in a concert setting. All that does is tell them that people can write ridiculously obscure stuff in concert notes and that classical music is expensive.
2. Use a parodist to convince them that they really know how good music should go first. Whether it's Allen Sherman's brilliant work for the Boston Pops or it is Peter Schickle's presentation of "PDQ Bach", music done badly is laughable, and points out that they ALREADY KNOW BETTER how it should have gone.
3. Then introduce the really popular classical music: the chase scenes from the William Tell Overture come to mind (not the beginning piccolo foolishness), along with many of the other pieces in this collection.
Serve beer. Encourage comment. The parodists are really good at getting things going (and "Variations on How Dry I Am" by Sherman is priceless).