title_body: Does pre-heating a puck on the group head produce more crema?
upvoted_answer: Well, I've heard a lot of "pro tips" and advice having worked around espresso machines for a few years. And I must say, this one is new. I have never heard of warming the coffee itself. I can't imagine that, at the temperature at which the brew head usually resides, many significant changes are made in just a few moments of warming the coffee.
  
  The closest I've heard to this is that some believe in the opposite. A fellow once told me to leave my spent puck in the portafilter if someone was going to be brewing again shortly. He claimed this helped retain heat in the portafilter. Though the science checks out, the results proved marginal and pretty much just made you the lazy guy using the espresso machine.
  
  So, in closing, I'd say that the effects of warming your coffee are probably negligible, and that you can get more crema more easily through other means (buying better quality coffee, adjusting your brewing variables, etc)