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Make sure you keep your braids flat, and the tension even. Don't make the braids too tight; that tends to create the "bowl" effect.
While you're braiding, you want the braids to be tight enough to hold their shape, but not so tight they start to have a rounded appearance. When coiling, the braids MUST stay flat, or the rug will start to bowl. If at all possible, sew on a large, flat surface, never in your lap.