Be sure and put the seal in square, matt. If you use a press, or a vice, to put it in it's pretty easy to keep it square. If you have to just tap it in, check it for squareness constantly, and use very small taps in just the right place. Examine the crankshaft right where the seal's lip runs against it. Worn out seals usually have been worn by either metal filings from the crankcase, or dirt from the outside. This often wears a groove in the crankshaft. You can't remove the groove, but you can put the seal in slightly more or slightly less than where it was before, so the sealing lip runs on a fresh bit of crankshaft.
Also, check the fit of the lower main bearing (which is a bush pressed into the timing cover) on the crankshaft. If it is a bit slack, you aren't giving the seal much of a chance: they can't deal with wobbling shafts. If the bearing has any wear, replace it while you can.
