What Paul810 said. We use carb cleaner spray to do it with. I run 12.5" wide tires on 7" wide rims (not recommended by tire or rim manufacturers), this keeps the tire on the bead at lower pressures. I've run mine offroad at 5 psi without losing a bead.

Also, if you have trouble seating a bead you can hook a ratchet strap around the circumference of the tire while airing it up. Just remember to remove it once the bead is set, before you air it up very much.

And be careful when attempting any of these things. Air pressure can do very bad things if you're not careful.