If your leak is between the seal and the housing it helps to sand the seal on the inner edge to help it start in the housing without catching a wanting to bend. Also shown is my homemade seal installer made from an old bearing inner race and a differential cross shaft that I made because I'm way too cheap and have way too much time on my hands.
If the leak is betwee the seal and the axle, as a last resort you might want to put a speedi sleeve on the axle seal surface. CR/SKF #99149 works good and can be found fairly cheaply on on the auction site that we all love to hate.