1/ Alcohol. Ok, so it's stating the obvious. But anyone who's tried to speak French will know how a ballon de rouge or four can turn you into Proust with beer-goggles (also known to improve singing ability, even in the most hopeless cases). Although in the case of Polish it may simply increase your confidence to the extent that you start adding -owac and -owany to the end of English words and conjugating everything in the genitive just to hedge your bets.*
If all else fails, you can always sit and translate the label on the back of the bottle (for 'translate', read 'bug your Polish friends for translations of...').
Warning: any vocabulary gleaned in this manner will almost certainly be forgotten the next morning.
2/ Biology. Remember those girls who came back from their Erasmus semester with a photo of some sloe-eyed Sicilian in their purses and a fluent command of the Italian idiom? Fraternizing with the natives is a long-established method for acquiring vocabulary without trying too hard, and you may even enjoy yourself. For fairly logical biological reasons, this works much better for girls than for guys. The irony therein is that Polish women, without exception, all look like supermodels**, while the guys look, well, pretty similar to British guys (socks under sandals, terrible haircuts and so on).
Caution should be exercised when using vocabulary acquired in this manner in a professional context.
3/ Crime. Have something stolen. As yet (touch wood) this method remains untested in Poland, but was experienced in a bizarre way involving a rented bike in Italy (bizarre because said bike was returned three days later when the end-of-term party season finished. Northerners are Weird). Following the theft you are compelled to go down to the local nick and attempt to communicate. You then have to pretend not to hear the Entire Station giggling like big girls because your driving licence ID gives your first name as 'Miss'- which, everywhere in Europe except the UK, designates a beauty pageant entrant.
You will never, ever forget the Italian word for a bike lock.
4/ Babcia. Rent a sofa from someone's grandmother in Nowa Huta. This may also be a direct consequence of quarter-life-crisis financial angst. There is No Chance that she'll be speaking any English to you, and you'll have to pick up one or two Polish words otherwise you won't get any dinner. As an interesting side effect, you will learn how to interpret the babcia growl, commonly uttered in queues if you complain that they may have pushed in front of you. Equally common on the tram: if accompanied by brandished umbrella, relinquish your seat Immediately.
5/ Sport. Go jogging in shorts. Very quickly pick up rude things to shout at lorry drivers.
Again, this is generally one for the girls, although you'd be surprised.
And see cautionary note to no. 2.
Interestingly, if you want to steal something on the Bvd de Clichy, this is probably a good way to get away with it.
To be continued...
**Although she will spend the next fifty years metamorphosing into a fierce babcia with a vicious umbrella- see point 4/. Is there any section of the Polish demographic I haven't managed to offend yet?