Apostrophe S

I'm going to try and unload a few posts that I had started but never finished up.

For some reason, the usage of the apostrophe s ('s) tends to be confused a lot in English. For example, just the other day, I was in a hospital and saw a sign that read, "Doctor's Only" for an area that had restricted access.

I thought to myself, "I wonder why it is so hard for some people to use properly, it has never been a problem for me."

We just use the apostrophe when someone owns something (Blogman's post) but we do not use it when it is time to make plurals (thousands of Blogmans). Trickier is the its/it's convention, when "its" is posessive and "it's" is for "it is." Tricky, but nothing that the average person can't handle.

Then I kept thinking, that isn't so easy. There's also another wrinkle. What is proper usage of the apostrophe is when it comes to plurals of abbreviations. For example, do I use the apostrophe if I want to say, There aren't that many DVD's that I want to own" or "How many RBI's does he have?"

RBI is a tricky one, is the plural of "Runs batted in" RBI or RBI's?

Which reminds me also of the word "moose." What's the plural of moose? Two moose, or two mooses?

Maybe this stuff isn't as easy as I thought.