I am constantly reading things written by apostrophe abusers. Whether they don't know the correct usage of the cute punctuation mark or they are simply too lazy to use it properly, I find the errors endlessly.
Can the apostrophe be that elusive to the average person? The concept isn't that tough. If the following word would be "has", "is", "was", etc, then you use an apostrophe. If the word is possessive, you use an apostrophe.
The most incorrect use of the apostrophe is in the word "its" or "it's". This is one of the only places in the English language that the possessive word does not use an apostrophe. So, if you'd say "the apple's seed", the correct apostrophe use if you change the word "apple" to "it" is "its seed". You would never use "it's" here, as that would make the phrase read "it is seed" and not be possessive.
If you are referring to a person's possession of something, you'd say "Bob's apple". If you are talking about a group of people, you never use an apostrophe unless everyone in the group, or the group as a whole, possesses something. For example, you'd say "Happy Jacksons" when referring to the group being happy. The name "Jackson" doesn't need an apostrophe, as it is a plural and not a possessive.
Enough ranting about that. Just remember - when in doubt, sound it out. This means that whenever you are unsure, get rid of all the contractions and use each word as it should be. This should help you figure out where to put the darn apostrophe.
