If you want to do things "by the book", you need the CELTA (4 week course, about 1000 pounds) and a degree - any degree.
Warning - before coughing up £1000 on a CELTA course, check out the competition. Currently natives are offering to teach in big city (ie expensive) Wroclaw for 30 an hour and now tonight someone's slashed it to 20 zł an hour for one to one tuition. You have to be very sure the CELTA will lead to a teaching job in a decent school or one to one before unloading that sort of cash into the sweaty palms of the colleges who may paint a more rosy picture of things than the reality. £1000 on a course means you have to earn about 5000 zeds to break even. If teachers are now happy to do it for 20 zeds an hour - well, you do the math.
Not saying it's impossible to make a go of it - but it's not as easy as it used to be. Add to that the talk of the Polish economy deteriorating/slowing down for Q4 2012 and possibly beyond, and a proper business plan needs to be in place before committing big bucks to CELTA or anything else IMHO. On the positive side, some posters on other threads or a small number who have in recent months PM'd me here on PF maintain they are doing very nicely and have more students than they can handle even sans CELTA. Whether that's word of mouth or the luck factor I cannot say. (None of them are in Wroclaw, however.)
You can also draw your own conclusions from the fact that an ad for a native speaker at a school right out on the edge of Wroclaw has drawn over 60 visits/hits in a couple of days. It's a quite hard to reach place, no salary is stated.