The Things I Learned in Fiction Class

Try to avoid referencing people by their physical characteristics or professions

What do I mean by this? 

BAD: 

"Hello," said Damien. The brown-haired gentleman sat down in a chair. 

"Hi," said Jim. The blond leaned back and closed his eyes. 

"Do you want something to eat?" The brown-eyed man held out a waffle to the blue-eyed police officer.

Were you confused reading that? Because I sure was confused writing it!

Your reader won't always remember who had what hair or eye color. It becomes confusing and unclear who is doing what. We have names and pronouns for a reason. Use them. The less your reader has to think about who is saying or doing what, the more they will be able to focus on what's important: your story.

BETTER:

"Hello," said Damien. He sat down in a chair.

"Hi." Jim leaned back and closed his eyes.

"Do you want something to eat?" Damien held out a waffle to Jim.

Now we can actually figure out who is doing what. Whew!

But what do I do if I don't know my character's name?

Pick something simple to refer to them by and stick with it. 

EXAMPLE:

The boy was young, so, so young, his skin thin and papery, his eyes huge with hunger. 

"What do you want?" said Scott.

"Do you have any food?" said the boy.

"Do I look like a traveling food cart to you? Get lost!"

The boy walked back toward the alley, scuffing his toes on the cobbles.

It is always very clear who is doing and saying what, even though the boy remains nameless. 

And of course, there are exceptions...

I once wrote a story where the narrator thought everyone else was beneath him. When I wrote from his point of view, I referred to other people as "the baker" or "the farmer" or "the cook."  I did this to emphasize that he cared about others so little, he did not even bother learning their names. Here, it was a narrative and stylistic choice not to refer to people directly by name for the sake of establishing the narrator's voice.