My little trick is to get one of those caps with the string attached (which I immediately throw the string in the junk drawer), and place it on the lens so that the loop is within sight of the viewfinder frame, or sticking straight up. Of course, real Leica caps don't come with that feature, so you have to GASP! use a third party accessory.
PF
Every time he looks at the lens he has a frown but when he puts the camera up to his eye he's smiling...I thought he would see the lens cap this time...and no on in the crowd tells him...