On TV, you've probably seen the stereotypical image of a secretary or administrative assistant, snapping her gum, talking on the phone in a bored voice, and filing her nails back and forth, back and forth. In real life, that's not what you want to do on either fingernails or toenails. The back and forth motion actually weakens your nails.
Starting from the outer edge where you've clipped your nail at a 45 degree angle, lightly file into the center of the nail. Repeat this motion several times until you've got a nice, smooth edge on that side. Then, do the opposite side from where you started, using the same gentle stroke and filing towards the center of the nail.
A square, or "soft square" is a good shape to create; making an oval shape gets you too close to the skin, again causing those ingrown nails. File so that you have a uniform look across, and then you're ready for color!