It's also really easy to get leaks at the oil pan gasket, especially if you use the multi-piece type. There is a one piece gasket that makes things a little easier.
FelPro makes a special oil seal for worn crankshafts with a double lip.
If you think it's coming from there, try to determine whether it's coming from the oil seal or the gasket. Take the front cover off the bottom of the bell housing. If it's coming from the seal, you should see signs of it on the front edge of the flywheel/flex plate.
Finally, don't forget that a bent or distorted oil pan flange can mess you up too. If at some point the pan was overtightened, it might be distorted enough to leak. Make sure it's reasonably flat.
Also, do not discount the possibility that it's the oil filter o-rings until you've looked really hard. The oil from there flows down and ends up at just the same place bad seals put it. It's hard to see the source sometimes. It's pretty surprising how well bad o-rings can masquerade as a bottom end leak.
