If you’ve ever created a gradient or drop shadow in Photoshop and had it “band” when saving (especially for for the Web), you’ll appreciate this tip: To eliminate banding, add a little Gaussian noise to the Luminance channel.
Do this: switch to Lab Color mode (Mode> Lab Color), turn off the a & b channels in the Channels panel, leaving only the Luminance channel visible.
Select Noise> Add Noise from the Filter menu. Click on the Gaussian button and use from 2% to 4% in the Amount field.
Switch back to your original color mode when satisfied.