When you're paying for drinks or a meal in restaurants or bars and you get the cheque, you should give the amount you have to pay and wait for the change. If you give the money and say "thank you" it will be treated as "keep the change" type of tip. This also goes for taxis but in general  tipping taxi drivers is uncommon but you can do that if you want. The average tip is around 10-15% of the cheque. Tipping is not obligatory, you can tip whoever you want to show that the service was good. 

Don't forget to tip tour guides and drivers too, but only if you are happy with the service they have provided.

Saying "hello," "goodbye" and "thank you" in stores, restaurants, hotels and so on is normal.

Drinking in public places is illegal (the police can give you a ticket) and also inappropriate.  But you will see many poorer men on the streets of Warsaw and other big Polish cities with open vodka/beer bottles. If you are standing outside a bar/club on Nowy Swiat with a glass/bottle this is acceptable as many people take their drink with them when they go outside for a smoke.

In Poland, ladies are always first and you refer to people who are older than you as Mr. or Ms.

Starting from November 2010 smoking at bus/tram stops (both under the roof of the stop and around it) is illegal and you can be fined for it. Smoking on streets, however, is very common and completely normal. You can't smoke in restaurants and bars unless there is a separate smoking area. 

 In many private homes and even parts of some museums you may be required to remove your outdoor shoes. Where this is the case slippers are provided for the visitor but in the busier museums be prepared to search through a large crate of single slippers if you feel the need for a matching pair and a good fit.