Have your eyebrows ready. Run an eyebrow comb to have all the hair going in the same direction to prepare the eyebrows for makeup application.An eyebrow pencil will allow you to fill in any areas that are too thin. You’ll have to change your thin eyebrows with a pencil to begin with. You should use light and short strokes if you want to fill in some light or sparse spots with eye liner. If you do this it will be clear that your eyebrows are coloured. You never want your whole brow to be filled with pigment. Don’t fill in any places other than where your brows will look unnatural or where it’s really necessary. Make sure that sparse spots are not noticeable. Make sure you don’t use your eye pencil to draw specified lines. This is unwanted. Be sure to make your strokes light and short in order for your eye pencil to create the illusion of hair. -read this article
For a dramatic effect, add eyeshadow or powder to your eyelids. Try using some eye powder or eyeshadow after you are done filling in the sparse areas of the eyebrows with eye pencil. With an angled brush, put the powder on your entire brow. The strength will stick to the pencil and all of it will blend together. Don’t allow the colours to fade. By allowing your make up to stick, you will guarantee that your sparse eyebrows won’t be noticed during the day. Using a little squirt of hairspray to keep them in place for the final touch on your eyebrows. Don’t spray right next to your eyes! The best technique is to apply it with your finger or a sponge. When you are doing this, take care not to ruin any of the makeup.
You would have a much better chance of avoiding having to deal with sparse eyebrows if you monitor how much you choose.