What Chubb said. If it's been on there a long time (and often even if it hasn't), you WILL need a rattle gun to get it off. Be very very wary of using a massive breaker bar, as it will stress everything - the chain, the gearbox shaft etc. With a rattle gun, even if it's fuck-off tight, you don't even need to hold the back wheel - the rattle doesn't even turn the shaft. It's a totally different approach to using brute leverage, and your bike will thank you for it.

If you have a compressor, air guns are dirt cheap (a mate just got one for this purpose for $30 from Bunnings? Supercheap?). Even a perfectly adequate compressor from the usual suspects is under $100 these days, so if you're gonna change them regularly, just buy the whole lot for a bit over a hungie.

If you don't have either and don't want to buy them, any bike shop, car mechanic, tyre fitter etc etc will have one. Or you can bring the bike over and I'll do it for ya.

I might add that my air gun is totally my favourite tool of all time!! I only use it for front sprocket nuts (and the trailer hitch occasionally), but every time I do use it, I giggle like a schoolgirl!! And a compressor is a tool of 1000 uses, once you have one.