If you're using "it's" or "its," remember this:
If you can replace the "it's" with it is or it has and have your sentence still make sense, you should spell it with the apostrophe.
If you can't, you shouldn't. "Its" is possessive, like "his" and "her": "That's its food."
And "One Week" by the Barenaked Ladies has the line "It's been one week since you looked at me," and the full spelling of that line would be "It has been one week since you looked at me."
So you can say "It's its itty bitty hit." (Well, you could, but only I tend to say sentences like that...)