Best way that I've used is with a buddy.
Your job is to open and close the pitcock, their job is to keep steady pressure on the brake pedal, pushing slowly enough for you to open and close the pitcock. Then, they let off the pedal and the cycle repeats.
Here's how one cycle goes in my garage:
Me: "brakes"
Them: "ok"
I then open and close the pitcock letting fluid and air out.
Me: "let up"
Them: "ok"
You are to open and close the valve quickly, so that fluid/air shoots out for the entire time it's open. Repeat the cycle until only fluid comes out for 3 straight cycles. Recheck the master cylinder after every 3 cycles. You can repeat this all day long if you just want to completely flush your lines.