Touch up your roots and only your roots. "I've seen so many clients who come in because they touched up their roots but accidentally dyed their ends, too," says Ionato. "The ends of your hair are more porous, so they suck up color very quickly." So quickly, in fact, that the runoff from rinsing out your roots can stain them. To avoid this, apply conditioner to your ends right before you rinse out the color at the top of your head. Or you could try this quick tip: Mix about one teaspoon of cornstarch into the dye to prevent it from dripping down through the rest of your hair.
Section, section, section. To avoid patchiness, create a middle part that runs to the back of your head and split the hair into four sections two in front of the ears and two in back. "Always apply the dye from back to front," says colorist Aura Friedman, of the Sally Hershberger Downtown salon in New York City. "That way the dye is sitting on the back of your hair the longest, which is naturally darker than the front anyway."