You'll have to go through a series of questions with both the prosecutor & defense attorneys. As previously mentioned, just allude to the fact that you've already made up your mind & the defendant must be guilty since they arrested him. The defense attorney will dismiss you almost immediately.
You can also take the opposing position that you wouldn't ever find someone guilty because of the flaws in the system. You can state that you don't like cops blah blah blah & you would never put someone in jail. That's not something the DA wants to hear, so they'll send you on your way.
Basically, just pick an extreme end of the spectrum & you'll be good to go. I've been summoned 3 times & this has worked every single time.