I just open my eyes wide (not necessarily 100% open, but wider than normal) and stare at the picture, without blinking. Forcing myself to not blink seems to be the key, at least for me.
After about 10 seconds or so, the picture gets weird. Kind of a mixture where most of it is blurry, but some lines are clear and sharper. At that point, I sort of look around trying to outline the lines that pop out in a 3-D sort of way. Usually once I figure out the main picture, the rest of it is easy to see.
Usually adjusting my distance from the picture (while not blinking) helps as well. On my computer monitor, 2-3 feet away works for me. That may vary for you, though.
Here's one more clue that might help you: The primary pic is the Pink Panther, and he's in the middle. There are other things as well, but I won't spoil it.