My English teacher, Mr Baines (of blessed memory), taught me a simple rule:

In the sentence, 'the dog's collar' the position of the apostrophe is determined by switching the sentence round such that the possessor of the collar (ie. the dog) comes last, ie. 'the collar of the dog'. The apostrophe then comes after the 'g' in 'dog'.

There are other rules, of course, like the distinction between 'it's' and 'its' as so ably described above.

I quite like rules for such things - as long as they are regarded as aids to communication and comprehension and not just as rules for their own sake. Nevertheless, one of my heroes, the English poet John Clare, rarely used punctuation and saw it as tyrannous (some of his poems, can be quite difficult to read and to make sense of, though).