we've done a tip bucket once, for our first gig we booked an upstairs room at a pub and to get some help filling the room we asked a water charity to join in, they did some promotion ffor us and then ran the door and took the money, the room was free (as long as the bar took enough) the only expense we had was to hire in some speakers for the evening as we didn't own any at that point, so we had a small bucket on stage with a label "tips to cover the PA Hire" we got enough to be able to put the extra back to the charity and all was good.
The gig was to get us some experience as we hadn't played together live (although we had all gigged in other bands, most of our friends turned up and it was a good night all round.
Matt