How do I prevent banding in a gradient

To prevent the color steps in a gradient from banding, or showing a harsh edge between areas of differing color, the simplest solution is to apply Noise to the gradient. A little goes a long way, however. In 5.5 or lower, use a Noise setting of no greater than 7. In 6.0 try to stay under 2%.

Photoshop can automatically do this for you. Check Dithering when selecting the gradient tool. It will introduce a little noise for you.

These processes break up the colors by introducing pixels in the gradient area that differ slightly from their neighbors - just enough to prevent a harsh edge developing when the image is color-separated for printing.