Tell Them Apart
These are differences between Mary-Kate and Ashley, and some of them can be used to tell them apart.
Their Hair (Currently)
After a liftetime of looking exactly the same, Mary-Kate and Ashley cut their hair different a few years ago. It has grown out and gone through changes since then, but it is still different. Currently, Mary-Kate's hair goes down to about a few inches past her shoulders, and every time I've seen it, it has been straight. Ashley's hair is several inches longer than Mary-Kate's. She wore it curly all the time for a while, but lately she wears it straight sometimes, too.
Their Freckles (Younger)
Mary-Kate has a freckle on her right cheek, and Ashley has one right under her left nostril. Really, this only helps if you're watching videos and movies from one they were younger, because they always cover up the freckles now and you can't see them.
Their Preferred Hands (Always)
There's not much evidence of this in movies/videos/etc., but it's a difference between the twins. Mary-Kate is left-handed, and Ashley is right-handed. This can be seen in Our Lips Are Sealed when the girls are on Victoria's father's boat for the first time. When they take the fish eggs from the waiter (before they find out what they are), Ashley holds hers with her right hand and Mary-Kate holds it with her left.
Their Height (Currently)
Ashley is currently two inches taller than Mary-Kate. This isn't always apparent, but in some scenes in movies and pictures, Ashley looks quite a bit taller than Mary-Kate.