I'm assuming you've tried the usual bits of large section angle iron and wooden wedges in association with the application of force to help persuade the beads of the rim, but mention them only because you dont. If you can borrow one, or know of a tyre company which has one, of the (dont know the right name) 'hammer levers' they work well. (Assuming you are built like a powerlifter). I'm not, so I felt lucky that I found a tyre company that was willing to have a go. They did one but the guy knackered himself so deferred on the others. They did suggest the aforesaid wedges etc and plenty of lubricant on the beads overnight. This did work for me, doing it when I felt like it, i.e. when I'd got my breath back . Goes without saying that once you've got them off to give the rims a bloody good clean up so that it is a lot easier next time. If one side is particularly stubborn then take the other off, give it a good clean so it will come off easily next time and then, sounds silly, but put it back on again. Turn it over and use the wedges etc on the stubborn side using the first rim to help support it. I've always found it is always easier to get one side off if the other is in due to cutting down the movement etc. Once you have a bit of movement on the bead it makes all the difference. I also made myself up some 4 foot long tyre levers from some suitable rectangle section bar, welded a stiffening piece in the middle away from the business end to cut down on flexing. These have proved very helpful too.