If the trans is in the car, I've always used one of the aluminum discs that come in a wheel bearing/bearing race install kit. I smear a light film of white lithium grease around the OD of the seal, and tap it in. It's tricky because if the trans is apart you can make sure that the seal is perpendicular by looking at the inside; but with the trans assembled you almost have to guess. Obviously, if you use this method, put the flat side of the race installation disc against the seal.
Harbor Freight sells this kit...
