If you use liquid foundation, you're better off applying it with a sponge rather than your fingertips. Your fingertips may spread bacteria to your face, which can cause breakouts. Makeup sponges also provide a more even application.
Don't use too much foundation, though, because it will settle into any nooks and crannies in your face and make them look even more pronounced. Experts recommend dipping the sponge lightly into foundation and dabbing it onto any splotches or discolorations first. Then, dampen the sponge and apply the foundation to your entire face. Take care to blend it nicely into your neck.
Also, clean your sponge regularly to avoid introducing bacteria to your skin. Yuck!