One of my biggest grammatical "pet peeves" is the misuse of its and it's. Examples of this little word being used incorrectly are everywhere!
It seems so simple to me, and I want it to become simple to you, too! The only time you insert the apostrophe is when you are using the word as a contraction for "it is" or "it has." There are NO other times when it is appropriate (or I could have said - when it's appropriate) to add the apostrophe.
If you cannot read the sentence aloud, substituting "it is" or "it has" for "it's," then you've used it incorrectly!
Examples:
It's cold outside today. (It is cold outside today.)
It's his favorite sport. (It is his favorite sport.)
It's been a long time since I've seen you. (It has been a long time since I've seen you.)
In all other cases, use its, with no apostrophe, to show possession. Its is a possessive pronoun (belonging to it). You are replacing a noun with a pronoun.
Examples:
When you show possession using a noun, you DO use an apostrophe: The kitten's tongue was rough when it licked me.
When you replace the noun with the pronoun, you DO NOT use an apostrophe: Its tongue was rough when it licked me.
Other examples:
The sun's rays were intense. Its rays were intense.
The sailboat's length is 42 feet. Its length is 42 feet.