What is a way to remember the difference between "saw" and "seen"?

Seen is something you've already seen and saw is something you want to see

Saw is the past tense of a the verb see. It forms the simple past, which used to express an action that has started and finished at a specific time in the past. 

Examples:
> I saw Star Wars yesterday.
> We saw the parade this morning.
> He saw her jogging on the sidewalk in the afternoon.

Seen is the past participle of the verb see, and it is used to form the perfect tenses; present perfect, past perfect, and ect. If this sounds confusing, don't worry. I'll explain everything down below. 

The easiest way to tell which word is correct and which is incorrect is that seen must have alongside it a helping verb. Past participles cannot appear by themselves in a sentence. 

Examples:
> I saw the movie. (Correct)
> I seen the movie. (Wrong)

Instead of past participles need what we call helping verbs to form the perfect tenses.

Examples:
> I seen the movie. (Wrong)
> I have seen the movie. (Correct)

If you see the word seen all by itself, you know a mistake has been made.