We pay babysitters $1 or $2 per hour over the going rate, and usually round up to a whole hour. The difference on a typical evening out is an extra $5-$10, which in the total cost of the evening isn't much.
Not surprisingly, we can usually find a sitter in two phone calls, if not one. (Text messaging is a great advance in babysitter scheduling. I swear the process only averages 7 minutes regardless of the time of day.)
I try to treat vendors the same way. They are much more willing to help me out when I'm in a pinch if I've been nice to them.
Constantly grinding everyone for the lowest price and most favorable terms is smart in the long run, but I suspect its expensive in the long run. Its hard to find help when nobody wants to take your calls.