if you step down on one side of the rim, preferably with the tire already mounted on that side, it will give some extra slack to slip over the tire on the opposite side.
also, rolling the tire on with both thumbs helps. Think of how you place your hands on the keyboard, now simply copy that over to the tire, and then roll your wrists, like you would turn a car key, but with both hands. The valve stem should be at 3' or 9' while doing this.
If they're still hard to mount, double check that you have the correct tire size and if possible use a narrower rim strip. Fat rim strips tend to make tires harder to mount, because they effectively increase the rim size.
Last edited by AEO; 11-29-11 at 06:55 PM.