Many people obsess over crema. I don’t get it, but having freshly roasted coffee makes a huge difference in crema. With fresh beans, I usually have 1/2 oz of coffee with 1 oz of crema (~1/2 in). With old beans, I get 1/2 oz of coffee with 1mm of crema. So if you like crema, use freshly roasted beans.
I’ve also heard that robusta (as opposed to arabica) is often added to espresso blends to enhance body and crema. It certainly does!. After I stopped the pull, the basket was hissing as the more crema came out. I had 1/2 oz of coffee with 2.5 oz of crema. After the pull, the crema was solid enough to have peaks where it had dripped down.
I got S. O. Espresso India Sitara From Paradise Roasters (http://www.coffeereview.com/review.cfm?ID=1206), and I ground and pulled it 5 days after roasting. In my experience, there’s a noticeable freshness difference 10 days after roasting, so this would have been square in the middle and the extraordinary crema couldn’t be attributed to post-roast outgassing. Of course, I’ve only tried this once, so it could have been a fluke.