To create (and maintain) the perfect brow shape, try penciling in your preferred shape before you tweeze. If you're uncertain about the shape, stop by a cosmetic counter and ask an associate to pencil them in for you. Also, stay tuned for my next post: a quick primer on brow shaping (complete with an easy to follow diagram!)
It's a good idea to make sure the rest of your eye is clean, you don't want to introduce any ickyness when you tweeze. Stand back from the mirror and draw in your brows quite heavily- since this is an aid to shaping and not for wearing outside of the house, you'll want to be able to see where you put the pencil. When you close in on your mirror, you'll be able to see exactly where you need to groom- only pull hairs that are NOT covered by your pencil.
If your brows have never been shaped, or are very grown out, follow the same technique as above, but use a white or nude pencil to draw over the hairs you'll pull. This will give you a more clear idea of what the shape will look like, and will keep you from over tweezing!