A couple things I have discovered. Very important to have a high end grinder. And I have also noticed quite a difference in my grind when I change coffee type. I usually recalibrate when opening a new bag. I use about 14g of ground coffee and should get about 2oz of esspresso in a 23 second pull. If I get more than 2oz my grind is to coarse and less too fine. My experience is there is more tolerance in the tamp than there is in the grind. The 23 seconds is something I read on coffeegeek.com and it has worked well. Pulling longer seems to make the coffee a little bitter. Coffeegeek is a great source and they wont bore you with a bunch a tea talk......

Most good coffee roasters have an espresso blend whick is a dark roast of aribica beans. I try many different roasts and have some personal favorites (Back Cat is very good, Cafe Artigiano in Calgary/Vancouver) but also find Starbucks Espresso blend to be very acceptable.

Hope that helps a bit.