On the stereogram of the chameleon, try this (not all stereograms have this, but they can help).
At the top of the image are two dots. Relax your focus and look "through" the image until the two dots appear as three, with the ghosts of the center dot aligned. Then shift your gaze down to the picture itself, and the lizard should pop out in a second.
It also helps if your line of sight is as perpendicular to the center of the image as possible, they don't work so well at larger angles.
What's funny is my eyes will force that effect even where it doesn't exist; for instance if I stare at a large block of undifferentiated text, the words will rise off the paper and the paper will recede.