The most effective way to repair too-sparse eyebrows is to simply leave them alone. Stop waxing or threading the brows, and use tweezers only to remove stray hairs that grow outside the area you want filled. Using castor oil or a product that stimulates hair growth may also help. However, for some people, hairs that have been plucked too many times never grow back.
Eyebrow experts suggest stimulating growth by rubbing a small amount of castor oil directly onto the brows each day. Some also swear by using products that prevent hair loss, such as Rogaine. Applying a small amount of the product every day may help brows fill in. Use a cotton swab to apply these products to the brow area only, and stop if you experience any redness or irritation.
Some have also found success applying Latisse, a prescription product designed to thicken and lengthen eyelashes, to overly-plucked brows. However, your brows may thin out again if you stop using Latisse.
Even if you can grow in the patchy parts of your brows, it may take as long as several months or even a year or more before you see fuller brows. Until your brows meet your satisfaction, fill them in with eyebrow pencil or powder.