There are just a few rules.
1. Thicker is more youthful.
2. Brows should be as long as possible, so make sure that they do not land within the line of your eye. Does that make sense? Mine are not long enough on the outer corners, so I add eyebrow makeup to make them look longer.
3. If they are thin or have holes, use make up to fill them in. I recommend a shadow for this.
4. Do not pluck them all out and draw them on, especially in a different place than they naturally occur.
5. When in doubt, have your eyebrows professionally threaded or sculpted at a salon after growing them out for about 6 weeks. Then, maintain your shape!