Making your own hats allows you to show a personal style and coordinate a complete look. There are patterns for hats, but designing a hat yourself may be even more appealing for those with a creative bent. There are several ways to stiffen brims of a sewn cloth hat.
Buckram is an extremely stiff, woven fabric which is usually made of cotton. It's often stiffened with starch, allowing the hat to be formed on a block or other shape, then dried in place. Once dry, buckram retains its shape. Use buckram in a brim as you're sewing, then form it to the shape that you want the brim to have. This stiffener works well for period hats such as sunbonnets, costume hats and baseball caps.
Another way to stiffen a sewn hat is to use many layers of fabric in the area that you want stiffened, then stitch the layers together with several closely-set rows of stitching. For example, for a bucket hat, use two layers of fabric for the main part of the hat and five or six layers for the brim. Stitch through the brim layers in several parallel rows. This method stiffens the brim slightly, while still giving the flexible look popular with bucket hats.
Stiffening solutions, or sizing, come in spray or brush-on varieties. It's also possible to use a spray starch. These are particularly effective for soft fabrics, such as felt. Follow instructions carefully on sizing materials, since some require that hats be completely dry before using, and some work on wet fabric.
Another way to stiffen a hat brim is to use stiff interfacing. Standard interfacing used for collars and facings will work, but won't provide as much stiffness as craft interfacing. Look at fabric and specialty craft stores for interfacing made for bowls and purses. This product also makes stiff hat brims.