Such as at a restaurant or club? Some people appearantly are good at slipping money to the maitre'd or staff to get a better table (or any table) in a restaurant, or to skip a line at the club, or to get some other special attention somwhere. I have never done this, and even if I wanted to, I have no clue much would be adequate.
If you've ever done this, tell us: Who on the staff did you slip the money to (e.g. maitre'd, hostess, waiter, etc.)? How did you slip them the money (e.g. palm of the hand, pointed to the top of the wait list with the money in hand, etc.)? And what is a good amount of money?
I have never done it. But while working as a hostess years ago I have had someone pay me $20 to get bumped up on the waiting list for a table.
Happens pretty often as a bartender-- usually it's something along the line of getting a tip with a promise of more if you make sure that certain people (business associates, etc) are well taken care of. Sometimes, when the lunch guys would bring in their international partners, I could make REALLY good money.
Personally, I have tipped hostesses to get seated in sections with specific waittress/bartenders (most places try to evenly disperse new customers, so sometimes you get a random spot/person-- and I'm picky! When I find someone I like, I want to keep giving them my business!)