Simply use your hand and thumb to squeeze them together - all around the tire except for the immediate area in which you are working to get as much free play as possible. I have been able to mount the old type of Specialized Armadillo (extremely stiff tire) without any tools using this technique, and regularly mount alll other tires - 700x25's - without using levers.
Several years ago I realized this could be done by watching a guy in an LBS slip on some difficult tires just as easy as pie without levers.