One of the formats that has never worked for me is the practice of going around the room in turn - absolutely uncontrollable quality-wise.
On the other hand, my favourite club works on the basis starting both halves with 2/3 selected performers, then a free for all - doesn't work either.
I sat in company with one of Ireland's finest singers one night - he is an extremely retiring individual and, because of this, he sand one song all night.
An observant and positive MC (or fhear an tí, in Irish) holds he key to a good, balanced evening, in my opinion.   
"Sonny and Brownie can have the second half..."
Is that all Al - a bit mean?
Jim Carroll