This question mentions one factor in crema production to be the choice of beans. I have tried many types (brands), all approximately equal in darkness of roast, and had some wildly different results. Why would this be? I'm sure the beans may be grown in different areas, but are all (probably) arabica beans. There even seem to be a lot of variance with different product lines in the same brand. Can someone explain why, or what I can look for in the future?
As a former master roaster at a local coffee shop, I can attest to the importance of tamping the grounds. The link in Dinah's post is has great info about making a great shot. At the coffee shop, I used to make the trainees tamp their espresso on a scale until they got the feel for it.
When the espresso is properly ground and tamped, the shot should draw in a beautiful, crema colored "mouse tail" (in a fine, steady stream that kinda "twitches" slightly).
The question you linked to has some answers about which type of beans to use (Brazil, arabica, robusta) but equally important is bean quality including/especially: freshness.
For crema, you want fresh beans, a good machine, a good grinder, and good technique. This page has great advice on crema: http://www.coffeekid.com/espresso/cremarule