Why is there so much crema on my espresso? I buy fresh beans of lighter roasts, grind a few minutes before extraction, pull a double shot normale, and end up with a lot of crema, I mean a lot!

Right when I'm done, the top 90% of the shot is crema moving around like the head on a fresh Guinness. After it dissipates for 30-60 seconds, I'm left with roughly a centimeter of crema of light color.

Crema is a pretty bitter flavor, so I wonder if there is something I'm doing to cause this type of extraction, and what can I do to improve on my technique?
Crema is basically carbon dioxide fizzing up the coffee oils. By the description you're giving, it is likely to be too much. Your beans could be too oily and/or too fresh. It could be a combination of the type of beans / blend / or the way it's roasted. 

Just as a guide, it is best for the beans to rest for around about a week after it's roasted. And if your beans end up soaking in oil, that is not good!