Sponge: Mix together the starter, the water, and 1 1/2 cups flour in a reasonably sized bowl. A slightly bigger bowl is always better. After all, this sponge is supposed to rise, and much as you want it to rise well, you don't want it to rise out of the bowl! (This is also something to watch out for when you feed your sourdough starter.)
Cover the bowl with a damp cloth. If the cloth is dry, then the top of the sponge will dry out slightly as it ferments. Let it sit until foamy. The time of this is bound to vary considerably, depending on the starter. With my starter, it usually takes about 15 hours to get it really foamy. However long it takes, don't let it sit for too long. Too many cookbooks say to let it sit for a time that's too long, and by then the yeast's activity is way down, and the lactobacilli damage the gluten. If that happens, the bread almost certainly won't rise, at least not as much.
Dough: Add the salt and the rest of the flour. Knead well. In the middle of kneading, wash out the bowl. This gives the gluten in the dough a chance to relax, and will make kneading it easier. Let rise in a greased bowl, covered by a damp dishtowel. When it's finished rising, punch it down. How long it takes for the dough to rise depends on your sourdough starter. Test the dough to see if it's done by prodding it with a finger. If your finger hardly dents it, it isn't done. If it leaves a depression that doesn't start filling in, the dough is ready for the next step.
Lightly grease a baking sheet, or sprinkle it with cornmeal. (I prefer the texture of the bottom crust with the cornmeal. Also, the baking sheet is much easier to clean.) Shape, place on the baking sheet, and let rise again, still covered. Just before putting it in the oven, slash the top of the loaf with a sharp knife. This is more than just decorative; it allows the bread to rise better. If you don't slash it, the upper crust could lift off the bread (called a "flying crust"). (It does take some practice to slash the bread well.)
Bake in a 425 degree F oven for 25 minutes. Let cool on a rack before cutting, or the bread won't be fully cooked inside.
Egg-Onion Sourdough: Before you add the salt to the sponge, mix in 2 eggs and 4 tbs. dried minced onion. This recipe produces a much larger loaf than the basic one, so you may want to divide it into two loaves.
Oatmeal Sourdough: Replace the water you add to the sponge with 3/4 cups rolled oats cooked in 1 cup water. The oats don't make the loaf heavier than usual. This bread has an interesting taste, but it doesn't have a strong oatmeal flavor. The crust is wonderful, dark and crisp. The oats aren't very visible in the final product.
Cheese-Pepper Sourdough: Add two oz. shredded gruyere and ½ teaspoons ground black pepper to the dough, just before you add the salt. (I've always used processed gruyere. Unprocessed would work too, but it might not blend into the bread's crumb as well. The crust is the best part, so this recipe would be very good as breadsticks.) I bet cheddar or parmesan would be good, too.