I think part of the trick is to have a number of different styles up your sleeve. Its such a shame when people always sing the same type of song each time. They may sing it beautifully, but if its predictable then its hard to give it full attention. There are some people who are amazing the first few times you hear them, but then you forget to listen after a while because it all sounds the same. I know I was a major offender for quite a while - but I'm working on it.....
So - if you work on having a range of songs in different styles available to you, then you can 'play it by ear' and select whatever seems appropriate in the context of a given night.
My most non-obvious song so far is 'Bad Connection' which is an old Yazoo (Alison Moyet) song. Hammerite & I sing it together unaccompanied, Hamm does the spoken bit (the telephone operator) with her nose pinched, and I do the phone ring noises (except when she accompanies us on the mobile!!).

Kris