Before mascara, women used to make their lashes look longer and thicker by coating them lightly with olive oil or petroleum jelly. The added grease would make their lashes look darker, and the color change would create the illusion of length and thickness. Creating a quick, easy illusion still works, but instead of using petroleum jelly, use eyeliner instead. A very thin strip of eyeliner applied at your lash line will add dimension to your lashes and make them appear more prominent. It's a cheat, but a nifty one if you prefer a more natural look or just don't like the feel of mascara.
Here's another optical illusion you'll like. If you have blond or brown hair and use brown mascara, go darker. The color change will make your lashes look longer and more voluminous. Another option is to let your brown mascara dry and then apply a second coat to the underside (lid side) of the lash tips. You'll lengthen the lashes a bit more without much risk of clumps.