Here's a helpful tip for getting really smooth drywall seams. After you've laid the tape in a four inch bed of compound, apply another thin coat of compound with a six inch knife. Let it dry. Add another coat with a ten-inch trowel, feather the edges out with a knife, smooth it out again with the ten-inch trowel, and let it dry. Apply your final coat the same way, this time with a twelve-inch trowel. Increase the size of your trowel for each coat, and you'll be sure to get an invisible seam every time!