I agre with Groo's suggestions about knowing where to use them.

I have also used some thought provoking by usking people if they know names of knots I've already prepared.
Within that group of kots I have a Theif knot and a Reef knot.
Everybody will identify the Theif knot as a Reef. (If they know the name) I then get them to pull the Reef and show that it wont pull apart, and then try the Theif and watch there faces as it unravels.
Then I start to show them how to tie a Reef without the posibility of accidently making a Thief Knot.

It gets them thinking a they pay attention after that.

Also use rope big enough that they dont have to rely on finger nails to untie.