Quicker than applying regular nail polish.
Cut off half of the hairs of an old nail polish brush, you can make it as thin of thick as you want. Dip it in some black polish and draw a line down the middle of a nude nail. I like the graphic elongating effect of painting only half of the nail and leaving the moons exposed.
*Must check out: Liza Chloe's striped nails, who did this almost a year ago! and Jayne's thicker version