I used to struggle with this one. To add apostrophe or Not to add apostrophe? Finally I came up with this trick. Change every 'it's' and 'its' to 'it is'. If anywhere it doesn't make sense, change it back to 'its' (no apostrophe). If it does make sense, change it to 'it's' (apostrophe needed).
Effect or Affect?
I figured this trick out in college. When I told the class, my journalism teacher didn't get it, but some of the students did. So here it is: Think about the shape of the letters 'a' and 'e' when you write them. The 'a' is puffed up like a balloon, while the 'e' is deflated. So if the word is meant to affect someone or something else, then use the puffed up one (a). If the word meant to explain how someone or something being effected, use the deflated one (e).
Loose or Lose?
Ok, when you want the word 'loose' you can be a little loose with your 'o' (double o), but if you want the word 'lose' you have to lose one (one o).
Whose or Who's?
You can use the It's or Its trick. Or you can use another trick: "The 'e' belongs to 'whose' but not the apostrophe." and you will remember that whose is possessive, but who's is a contraction.
Who or Whom?
This word choice is going out of style. The exclusive use of 'who' in all situations has become largely acceptable. But if you want to be technically correct, you can use this trick: Think 'who me/to me' and whenever you are trying to decide which word to use, find out if the main verb is happening to the 'who' or if the 'who' is doing the verb. For example, "Who is talking to you?" or "To whom are you talking?" In the first example 'who' is doing the talking, but in the second example 'you' are doing the talking to someone else.
I write about, with, for, and around kids all day. (Well, maybe I do the dishes too. Sometimes.)