To make eyelashes appear fuller and thicker, focus on the base of your lashes when applying mascara. Before coating, push the wand right up against the root of the eyelashes and wriggle it from side-to-side, eventually working your way up to the top of the lashes (this will also prevent clumping!).
One of the best mascaras for creating great volume is the CoverGirl LashBlast Mascara ($6.99 Target). For the British gals who can't get their hands on CoverGirl, I recommend the Rodial Glamolash Mascara XXL (£24.00 rodial.co.uk) as a great alternative.