Check Engine - Evap system leak detected - list of fixes?

[lurky]

Hi, Ive got a 2007 Boxster with 102k miles. The check engine light went on and I was get the error code: Evaporative Emission Control system leak detected

The last thing I want to do it take it in to the dealership, I know this be hard to pin down and they would x10 charge me for every step along the way to finding it, so I trying to address it myself.

A great poster here suggested that I replace the gas cap, and clean the metal filler neck's "lip" , which I did and then I cleared the error, but it came right back.

So what else can I try? Is there a sensor that I can replace? If you had this error, how did you fix it? Is there a series of fixes in order of most likely cause?