What factors affect espresso crema? For a given coffee and roasting method, what factors of the espresso making process affect the crema and in what way(s) do they affect it? By my own empirical evidence, I would guess that time since grinding, coarseness of the grind, and tamping pressure are the biggest factors.
From Wikipedia:


  The "crema" is produced by emulsifying the oils in the ground coffee into a colloid, which does not occur in other brewing methods. 
  


Using the right material from the start

Personally, I think this definition could use some updating. Crema exists because of the freshness of the coffee, in relation to it's roast. It contains the gasses we find from roasting, such as carbon dioxide and nitrogen gasses. You will notice that if you dig up coffee that is months old and brew it as espresso, there will be little crema. Bottoml ine, if you want crema, you need fresh coffee.

Extraction

The other factor in producing crema, is correct extraction. If you have a poorly extracted shot of espresso, it will be primarily water, with some seriously over-extracted coffee. An even extraction will produce the highest yield of crema.