Firstly, by demonstration - point out (using the clockcode, of course) any other aircraft that YOU see.
I quite often run a competition - if you see an aircraft before I do then I owe you a beer/£5 and vice versa - by the way, the instructor always wins!
That said I believe good lookout is an acquired skill and that nobody really knows how important it is until they operate in a (very) congested area and/or have an airprox!
Obviously teach them all the stuff about how the eye sees etc.