[susan53]

Persons is very unusual except in formal written notices or legal English, in set phrases such as Missing Persons, or used humorously as in Mesmark's example - the humour comes from the unexpected impersonal style in a situation where the speaker quite clearly knows who s/he's talking about. In normal everyday English we would almost always use people in both speech and writing, so the best rule for most lower level students is 1 person (sing) 2 people (pl).
Hope that helps!