I have a hard time knowing when to use "nan" vs. "an" (as prepositions with verbs of motion such as "ale, vini, soti, elatriye"). Any pointers?

Do you mean the prepositions AT, TO, FROM, ETC...?

If yes, then we'd use the Creole NAN.
ex:
1. Mwen t'ale nan yon fèt. (We went to a party.)

2. Nou prale nan legliz la. (We'll go to the church.)

3. Li te vini nan fen fim nan. (He came at the end of the movie.)

4. Nou soti nan yon gran fanmi. (We came from a reputable family.)

As far as AN (let's) is concerned, we'd use it imperatively as in AN NOU (non contracted), AN'N (contracted), or ANN (contracted)
ex:
5. Ann ale. (let's go.)
6. Ann soti deyò a. (let's go outside)
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