If you aren't listening the the Strength Coach podcast, now might be the time to start.
In episode 39, the "Hit the Gym with a Strength Coach" segment features Dan John. If you've enjoyed anything written by Dan John, listen to this interview. His interview starts at about 31:00 minutes in.
Strength Coach Podcast 39
In it he relates an interesting but simple concept - a standard workout/diet. He'll use the same workout and same diet each time he tries a new supplement or whatever else, and if he feels a change, he can pinpoint it to the supplement. Scientifically, that's isolating one variable. It's just one of the concepts you can pull away from this interview.