My first machine to make espresso was a espresso/drip coffee combo. Not knowing any better, I was merely pulling strong coffee and not espresso shots. Then I got a stand alone espresso machine, much closer to the real deal but still a far cry from what a coffee roaster like Temple Coffee could make in their store. Finally, invested in a Breville BES870XL and after many trials and errors, my espresso shots are fairly indistinguishable from what the pros at the coffee roasting shops make.

Getting the beans from a roasting store like Temple or Verve or Ritual (Sac/San Fran area) makes all the difference in the world for taste and crema IMO.

The amount of crema is definitely tied to air exposure so the quicker you can use up your beans the better (under 2 weeks or so from exposure to air). Additionally, its been my experience that I have to adjust the grind setting to 1 step finer if the beans have been sitting for a very long time, past the 2 week mark or so. I do this because at original setting, the extraction begins at a much lower pressure threshold than usual, leading to under-extraction and weak, watered-down espresso with little to no crema. It'd be nice to know if anyone has studied this particular aspect of coffee beans needing to be ground finer to compensate for age to maintain consistency.