I'm sorry to hear about your experiences then and that it has changed your views about Polish people. No one hates the Polish criminals more than the Poles themselves as they give us this kind of reputation abroad.
Unfortunately i've made similar experiences in Warsaw. I know it's not the whole folk to be blamed for it and it could happen in any other country but it did happen over here. Sorry guys don't mean to put you down.
It happened to me in Poland as well.. and I'm Polish ;)
Basically as a tourist, all the usual applies: Don't look like a tourist. Don't get lost in shady neighbourhoods. Watch for pickpockets in crowded places like metros.
Sorry to hear about it Joan.