By popular demand:
3-D eyes that do not use plastic domes to construct. These eyes are recessed, and they are an optical illusion to the viewers that the costume is "looking" at you no matter what angle you are viewing at. Use flat plastic, fun foam, or any material you think will work good for this project (use your best judgment).
Please note, this is just based on my observations
of masks with this technique, I haven't actually featured it in any of my masks yet. So credit goes to those who originally thought up this great technique!