Another common oil leak is the valve cover gaskets. If the valve covers are leaking, the first thing that you need to do is check to see if the bolts are tight. Over time these gaskets will shrink, and the bolts need to be tightened. If you need to replace the gaskets, you must take extra care to make sure you clean all the old gasket material off the cover and the cylinder head that it seals to. If you have a very large oil leak these two items will need to be checked.
Most motors have a front and a rear crankshaft oil seal.
Getting to either of these seals is normally the hardest part. On some motors with a two piece rear main oil seal this can be quite expensive. Most newer model motors use a one piece seal, so in order to replace this seal the transmission must be removed. So you will want to make sure you do it right. Most people don't know that as these seals get older they get very hard and they can actually cut a groove into the crankshaft.