"You've been following me," Ariel looked at Sheila, and she smiled.
Who am I talking about up there? When you over-use pronouns, you confuse readers. Learn the tricks of spotting excessive pronoun usage, and eliminate it from your writing.
He Said, She Said, They Said
Eliza looked at Mary. She shuddered before she spoke. "They're coming for us."
There are so many pronouns in the example above, no one can tell what's going on. Did Eliza both shudder and speak, or did Mary? Maybe Eliza shuddered, and Mary spoke. Or it could be the other way around. Maybe there's a third she involved in this mix.
The point is, I don't know. And you know what? I'm not going to try to sit here and figure it out. When I'm reading a book, the last thing I want to do is play the Match the Pronoun game. If I have to stop reading to ask "wait -- is that 'he' Marcus or Dave?" then you aren't doing your job as an author.
Grammar is always difficult to figure out, and the best way to keep your pronouns in check is through careful editing. But there is a hard-and-fast rule that I like to use that does help: don't repeat the same pronoun in a sentence. If there's a she, use it just once. Same with he and all the rest. And if there is any confusion at all, use a proper name. Sometimes, you might need to add pronouns to once sentence and take them from another. But I like to eliminate pronouns entirely when there's confusion.
Eliza looked at her. Mary shuddered before speaking. "They're coming for us."
Eliza looked at Mary, who shuddered before she spoke. "They're coming for us."
You have to be particularly careful of pronouns when two or more persons of the same gender are interacting, because tracing all the he and she stuff shouldn't be difficult. Make your book easy to read, and don't make reader play the pronoun game.