Although not very many famous poets write prepositional poems, they are a common learning tool used in classrooms. They help teach creative writing and proper usage of prepositions. They can also be a fun way to teach grammar in English as a Second Language (ESL) classes.
The rules of prepositional poems change from classroom to classroom. Some teachers expect students to start each sentence with a preposition, while some only ask for a majority of sentences to begin with a preposition. If you're a student, make sure to ask your teacher what his requirements are before you begin your assignment. If you're a teacher, make sure to clearly delineate what you expect from your students.
The first stanza of "Out of the Cradle Endlessly Rocking" by Walt Whitman contains many phrases that begin with prepositions, and may be used as an example by a famous poet. Examples are also available in chapter 23 of "Writing Simple Poems: Pattern Poetry for Language Acquisition" by Vicki L. Holmes and Margaret R. Moulton. Many teachers also post examples of their students' work online.
If the students are okay with sharing their poetry, read some of the students' poetry out loud to the class. Display it on classroom walls, and later on, if students forget the purpose of a preposition, have them look at their preposition poetry. The more fun they have with it, the more likely they are to remember it.