Getting Apostrophe's right is not all that tricky. Here are the basics from the Apostrophe Protection society
Take a visit to their page to see Apostrophe abuses that concern those of us who care about the correct use of the Apostrophe. There is also a little test at the bottom of the page for checking your knowledge.

The rules concerning the use of apostrophes in written English are very simple:

1. They are used to denote a missing letter or letters, for example:

external image apostrophe01s.gif
I can't instead of I cannot
external image apostrophe01s.gif
I don't instead of I do not
external image apostrophe01s.gif
it's instead of it is

2. They are used to denote possession, for example:

external image apostrophe01s.gif
the dog's bone
external image apostrophe01s.gif
the company's logo
external image apostrophe01s.gif
Jones's bakery (but Joneses' bakery if owned by more than one Jones)

... but please note that the possessive form of it does not take an apostrophe any more than ours, yours or hers do
external image apostrophe01s.gif
the bone is in its mouth

... however, if there are two or more dogs, companies or Joneses in our example, the apostrophe comes after the 's':

external image apostrophe01s.gif
the dogs' bones
external image apostrophe01s.gif
the companies' logos
external image apostrophe01s.gif
Joneses' bakeries

3. Apostrophes are NEVER ever used to denote plurals! Common examples of such abuse (all seen in real life!) are:

external image apostrophe01s.gif
Banana's for sale which of course should read Bananas for sale
external image apostrophe01s.gif
Menu's printed to order which should read Menus printed to order
external image apostrophe01s.gif
MOT's at this garage which should read MOTs at this garage
external image apostrophe01s.gif
1000's of bargains here! which should read 1000s of bargains here!
external image apostrophe01s.gif
New CD's just in! which should read New CDs just in!
external image apostrophe01s.gif
Buy your Xmas tree's here! which should read Buy your Xmas trees here!

Note: Special care must be taken over the use of your and you're as they sound the same but are used quite differently:
your is possessive as in this is your pen
you're is short for you are as in you're coming over to my house

Pictures of those using apostrophes incorrectly for plurals - seemingly willy nilly. These are called the 'greengrocer's apostrophe'. See if you can see why. Perhaps they think they look nice!
There is a good poster about using the Apostrophe here.
To be fair - here is an argument for killing off the poor old apostrophe