Lately I’ve been pulling more consistent shots, thanks to advice in David Schomer’s book, Espresso Coffee Professional Techniques. He suggests leveling the coffee with your fingers, then tamping with thirty pounds of pressure, and finishing by twisting the tamper while applying a slight pressure. I’ve found this not only produces better shots, but also prevents coffee from sticking to the tamper.
As was pointed out in the CoffeeGeek forums, consistency is more important than achieving exactly 30 pounds of pressure. This is because the fineness of your coffee grounds must match your tamp pressure. You’ll know if you got it right, because the espresso will be oh-so yummy.
If you’re interested, you can practice tamping consistently on a bathroom scale. Just remember to wash your tamper when you’re done or all your coffee will taste like feet.