Unlike possessive nouns, possessive pronouns do not use apostrophes.

my, your, his, her, its, our, their

mine, yours, hers, ours, theirs

The difference in the way we treat possessive nouns and possessive pronouns is doubtless the reason so many people have trouble distinguishing between its and it’s in particular. You wouldn’t add an apostrophe to my or mine or his or her or hers — so anywhere you could substitute one of these other possessives (not for meaning but to see whether the structure seems all right), use the apostrophe-free possessive its. Anywhere you could substitute it is, use the apostrophe-containing contraction it’s.

The difference between its and it’s may seem trivial — hardly worth bothering with. But many people who are accustomed to making the distinction see this the other way around: once a person understands the principles of using apostrophes, the effort it takes to get them right is trivial.