Part of my scholarship requirements has me talking polymer chemistry to businessmen who last did chemistry at O Level, if that. When I give the talks I agree with the advice given by H.
Pictures are great. I use simplified images to explain points and it comes across a lot clearer than text.
Avoid technical jargon.
Use your non-specialist friends. I usually give talks to either my non-chemist girlfriend or one of my humanities mates. If they look confused then the presentation needs altering; their advice has been invaluable.
It can be a little annoying to simplify your work, but it is an essential skill and if any watching your presentation does have more than the average idea of what you're talking about, more than likely they'll ask a question or seek you out later, giving you a chance to really show what you know.