I never thought about considering the simile as a solution to deciding whether to use “like” or “as” in a sentence until I saw this remarkably content-free comic:
A simile is a figure of speech that compares two things (think noun) by saying one is like the other. Its companion figure of speech is metaphor, which compares two things by saying one is the other.
Simile: A donkey is like a horse
Metaphor: You, sir, are a donkey
I never heard of anyone in English class having a problem getting these right.
Deciding whether to use “like” or “as” is a different matter. And here’s the solution:
Are you comparing two things? It’s a simile—use “like.” The donkey is like a clown.
Is it about verbs or adjectives? Use “as.” The donkey is as funny as a clown (is.) He hit his head as he bent over. He’s sick as a dog (is).
(The “is” is implied; I put it in so you could see it.)