1: Make sure the rim and tire are really the same size; i.e. that you're not trying to put a 700c tire on a 27" rim.
2: A little soapy water on the rim edge can provide lubrication and help the tire slip over it.
3: Using the thinnest possible rimstrip will let the tire bead go as deep as possible into the rim channel and give you a bit more slack to get it mounted.

But - if it's that hard to get on in the first place are you sure you want to risk getting a flat and having to change it in a less convenient setting?