What kind of rims are you using?

The trick to mounting bike tires has to do with the portion of the tire farthest away from the last section you are trying to pry over the rim. You need to have both tire beads squeezed together in the lower center portion of the rim rather than on the bead seats. If you can make that happen, the last postion of the tire bead will slip over the rim with ease. The more pronounced center dip you have in your rim extrusion, the easier your tires will go on and off.