You should never have to use tire levers to mount tires IMO. I've mounted all sorts of tires that friends were struggling with and said were "impossible".
Here's a couple of tips that can help:
-As mentioned above, leaving the tires in the sun or a warm environment will help a lot if they are cold.
-Be sure to start away from the valve stem and work towards the valve stem. Doing this will allow the already mounted section to get to the deepest part of the rim valley where the diameter is the smallest. This leaves the most "slack" available for the remaining unmounted section.
-As the tire starts to get tight, make sure the bead of the tire is in the deepest part of the valley in the rim. Keep doing this, and you'll get a little "slack" where you're trying to flip the bead over the rim. I will do this 4-5 times on really tight fits, every millimeter helps.
-When you get down to the last few inches and have exhausted getting all the "slack" you can use a tiny amount of soapy water on either the bead or the rim area of the last little bit. A small amount will help a lot for the most stubborn of tires.
Good Luck!
