If you have been picking up girls before there is no difference in Poland than anywhere else :)
Not true as far as foreigners are concerned. Lots of Polish girls, especially in touristy cities like Krakow, know very well why many foreign guys come to Poland and will be keen to show you that they are not 'typical, easy Polish girls'. Some of them are really paranoid about this. Also, in recent years, many horny Italian guys (loveable guys that they are) have given Mediterranean guys a bad reputation. Girls are still friendly to foreigners, but not as much as in the past.
So, what can you do? Be humble, humble, humble. Also be playful and make them laugh, but don't come across as too horny, don't offer to buy them a drink and don't behave like a 'playboy' - you will send out the wrong signals. Don't talk about sex, unless she is a particular type of girl. And show some interest in the girl - many foreign guys just talk about themselves all the time, and the girls assume that they only want to fk them (which is true anyway, but I think girls like to have the illusion that there could be something more).
The 'technique' I like the most is just to be honest but in a funny way. For example, if you stop a girl on the street, ask her for directions, and then confess that you didn't need any help - you just wanted to stop and talk to her because you thought she was hot. Or just say something playful like ask a girl in at a club bar if she can buy you a drink because you heard Polish girls are very hospitable. Whatever is appropriate in the moment.
Daytime game is usually more reliable because there is no competition from other guys or distractions like loud music, their b*tch friends trying to pull them away from you, no "I must look for my friends" excuses, etc. However, you need to be feeling confident to approach them in the day.
Of course, Polish girls' behaviour is not always so predictable and there are exceptions, drunk sluts, etc.... but stay away from the sexy-looking girls who like to talk to big body-builder type guys with shaved heads - you're not their type ;)