Anything less than spotless portafilters and tubes spell trouble for your crema.
Why? Bacteria are getting into the coffee. (That's only good news if you're a gastroenterologist.)
Backflush with Cafiza - now and forever.
And don't leave a loaded group head on the machine, full of used coffee. Use a group head brush. Then wash behind your ears.
Fast extraction
The espresso flow should take between 22 and 28 seconds. Any less, and you won't get the oils out of your coffee - and it's the oils which form your crema.
Grind size too big? The water gushes through too quickly. Tamping matters just as much. Technically speaking, give it a 20 lbs tamp - then tap. A final 40 lbs tamp should be followed by a light "polish".
The water pressure needs to be spot-on too - at 9 bars. Oh, and keep that portafilter spotless.
Not-so-fresh
Would you serve stale bread? Then why serve stale coffee? The lack of crema is a dead giveaway.
Don't buy in bulk. Order small amounts of espresso beans, but regularly - to ensure ridiculous freshness.
Once opened, use the beans within three days. (Don't forget to store them somewhere cool and dark. It really does make a difference.) And always grind your beans strictly to order.
