By trial and error I discovered that I like to use a slow song containing whatever I want to learn. That way I can have some feeling success that much sooner, and avoid at least some of the frustration that comes from not being able to play new patterns (chords and/or picking) at desired speed.

The most notable example of this I can recall: As a 20-someting in the late 70's I figured it was time to master barre chords. Being a Grateful Dead fan at the time (okay, still am) I decided to learn High Times, which is in E and chock full of barred chords on 2nd and 4th frets. Yet it has a slow, rocking kind of beat that is very forgiving to beginners. While I was developing calluses on my index finger, it was even slower. I still use it as a warm-up.