Certainly start with something easy and finish with something strong.

Mary and I have a few other criteria, some of which may apply to others:
- mix songs and instrumentals to give her voice a rest
- mix songs with different instruments (e.g. not 3 concertina accompaniments in a row)
- We try to do one half with banjo tuned for G and the other half in other keys (A and C, with capo on it goes sharp). That way, retuning is only needed in the interval with a bit of luck
- insert a "disposable" item one before the end of each set so if you are going to run out of time you can drop that one without regret.
- Do a chorus song early on if necessary to gauge audience's chorus singing ability/enthisiasm. Possibly adjust reminder of program accordingly.
- make a set list. After the gig, edit it to reflect what you actually did and file it, so if you get re-booked you can avoid repeating too many songs.

Lots of other good ideas in the rest of the thread too....

Anahata