We look at songs that we have brought into the act since our last recording, as well as new songs we want to introduce.
Fifty minutes of songs in the key of Am, with guitar, pennywhistle and Bodhran, with an AABB verse followed by AABB chorus that repeated two lines from the cerse, about some brave lads lost when a ship sank, all at 60 bpm, in 6/8, and sung with great sincerity, and the same harmonies, would drive the listener to drink, murder, or suicide.
So we build a grid that compares various factors of the songs/tunes we are considering. We try to avoid putting many songs in the same keys, with the same basic subject, the same tempo, the same instrumentation on the same album. If we have a reason to put similar pieces on the same ablum, we make certain that they are widely separated on the recording. We try for as much variety as possible. We look at pieces that we think we can perform with a fresh treatment.

Oh, we also pick pieces that we can play with reasonable musicality.