Rear main seal
If you've read the clutch DIYs, there's been a small debate about whether to change the rear main seal if it's not leaking. Some people changed it when it wasn't leaking and now it leaks. So, you may want to go back and read some of those posts and decide depending on your mileage.
No seal lasts forever, but in my case I decided not to change it when I did my clutch around 100K miles. It was not leaking, but the rear main on my 540 had leaked, so under normal circumstances I was inclined to do it. I also had the flywheel removal tool. I think what made me reconsider was that 1) some people changed it and it started leaking after the car was buttoned-up, and 2) if I recall correctly if you're not careful you can tear the upper oil pan gasket when you remove the rear main cover. To fix the later problem you have to drop the entire front axle to be able to get access to change the upper oil pan gasket.
I decided to leave well enough alone since it wasn't leaking. Obviously, if it's leaking then there's no decision. Change it.