No real secrets. First off warm the tires, a nice hot sunny day works well. Leave them in the sun for three plus hours, rotating the sides to warm them evenly. This will loosen the sidewalls up and prepare them for the next stage. Liberal use of tire mounting lubricant/tire soap on the bead will also help plus you will need at least two people to gently "massage" the tire onto the rim using tire spoons. Good luck
Getting really stiff tires to seat once mounted can be fun too. In a pinch I've had to use the ether, flame and air line trick.