Always make sure you have lots of tip change on you so you can give exactly what you want and not risk having to try and get change back or just leave it having been ripped off.
Unfortunately many places, especially bars, pay low (or, so I've even, even no) wages as the staff get their money from the tipping culture as tipping is expected rather than discretionary for good service (I don't see the point, why not just put the prices up and pay the staff better to incentivise them to provide good service for extra tips on a service basis!).
In New York it was a challenge to get through the lobby without a clamour of bell hops trying to get hold of our bags for the tips. We were told by one upfront, when just asking for some advice, directions I think, that they work on a tips basis and what their minimum tip was.
In the taxi we got from the airport, we had a previously agreed fee given to us by the taximarshalls. We had to pay the road toll on top, fair enough, then gave what we thought was a generous tip only to be told quite rudely that it wasn't enough and a demand to pay extra.
We got the hotel to arrange one of their cars for us on the way back as it worked out a lot cheaper.
We had some time to kill before our room was ready and ended up in a small bar up the road where there was an Irish barmaid who was fantastic in explaining the whole tipping culture to us and what would be expected when, helped us a lot to factor in the additional cost.