I like to start with something rowdy, with an easy chorus, and finish with something totally daft.In between, I try to vary the content as much as possible, to keep the audience interested. I often finish a song with an intrumental - the most requested is 'The Man Who Waters the Workers' Beer' followed by 'Blaze Away' on my accordion.
It's an advantage being able to play a few instruments, too, though a pain to lug around!!
Very rarely do I work to a 'set list' - I tend to have a shortlist of probables, and wing it from there. That way I can allow the audience to 'choose' their preferred material, so to speak...