I hosted an Open Mike Night in a cafe in Germany a few years ago. The owner is a wonderful musician & there was a group of really talented regulars who made every Wednesday night a real treat. Luckily for us, there were always really good travelling street musicians whe were willing to pop in for a free beer on the invitation of either the staff or one of the regulars. I (non-musician, bartender/cook) sort of fell into the host position because 1) I was there, 2) nobody else was willing to do it & 3) There were several bottles of Grolsch previously ingested. There was a HUGE potted plant right in front of the mike (had to put it somewhere) & I just started into a smarmy, Vegas-style lounge thing "welcome to the Jungle Room" or some such crap & folks seemed to like it. We kept the sets short, encouraged different combos (didgeridoooo & accordian among some of the most memorable), & passed the hat at the end of the night & split the take among the players. Beware of egos, be diplomatic, be spontaneous, keep it light & keep it moving.