Get a cheap felt tip black eyeliner. Cut a little off the top to give a blunt edge. Use this to do tiny dots along your lower lash line instead of eyeliner, eyeshadow or mascara, especially for a natural look. It will give the impression your lower lashes are fuller and thicker by filling in the roots. Just three tiny dots at the outer corner is usually enough, but sometimes i do it all along to the middle of my eye.