A contest judge tells me that if there s more than one such error in the first page he s unlikely to bother with the rest of the piece Your best bet is to write for the contest judge, and be relieved if a permissive editor catches you out. Its is just another possessive. It doesn t need one either. But contractions do. It is a hot day becomes it s a hot day , in the same way that S he was not unhappy contracts to she wasn t unhappy .
Plurals are easy.
They never need apostrophes. Seven dogs are in the yard and the black one is scratching its leg. My dog s asleep at home , just about covers it. Repetition Watch out for words repeated too closely on a page. Then, either change them or move them.