Can't get tyre on / Tight rim

Has anyone tried just putting the tyres next to a radiator before you put them on? Also leaving a tube inflated inside a tyre for a while or using washing up liquid. I used to have a trials motorbike and if you think bike tyres are hard you should try changing them.

Regardless, ambient temp makes a big difference to how easy it is to put tyres on.

Oh yeah had a honda 125 mx bike myself, the 21 at the front was okay, but the 18 on the back, fuuuuuuuuuuuuu.