Common Oil & Tranny leaks

[fruitmeister420]

I've been slowly bringing my '83 300D back from years of deferred maintenance. Up to this point I have replaced...
all the belts
the crankcase ventilation hoses
the turbo air intake seals
air filter and isolators
flex pipe from turbo
broken radiator overflow
hood pad (missing when I got it)
shift linkage bushing
B2 piston & related bushings, seals, etc.
flex disks & carrier bearing assembly
fuel filler neck seal
brake reservoir grommets
injector return lines
front & rear brake pads (flushing lines)

Also I have flushed the PS system and replaced & repaired the filter

But one thing continues to bug me, and that is/are oil leaks and or transmission fluid leaks. So it seems to me that it would be handy to have a thread where you could review the most common sources for these annoying leaks and possible fixes for them.
For example the oil filler cap leaks which are so common on the 617 engines. In my case I ordered a new gasket, having read about some people who have used double gaskets to solve this problem. I believe that this may have solved the problem, but the oil vapor vent hose next to the filler cap seems to be leaking as well so I have to resolve that as well.
But to my point, is it best to start at the top and work your way down? And while some leaks are easy to fix, others seem to be a real pain such as the front tranny/rear crankshaft seal(s).
So what are some places where leaks frequently occur and how can they be fixed?