When I did mine my mechanic uncle told me to rub a very thin coating of the ultra grey permatex rtv around the outer edges of the seal before putting it in. I guess the theory being that it helps it slide in smoothly and then once it dries it also helps seal any tiny imperfections in the metal and keeps it in place.

Also are you using mitsu oem seals? The rubber quality is usually superior to the aftermarket brands.

98 Montero, 184k.