The College Board has published several paragraphs detailing what the readers will be looking for in answers to "paragraph response" questions. Take a look here. I don't have much to add to their statements, which I think are clear and useful. I will likely pass out their page-long discussion for my students to read during our exam review.
The two points I'd highlight:
(1) "It should make sense on first reading." Your students don't get to come to Kansas City in order to follow their exam from reader to reader saying "let me explain what I meant." You only get one shot -- do not miss your chance to blow the reader away with your logical arguments.
And thus, in your class, don't allow a student to argue about his score on this kind of problem. If it didn't make sense to you ON FIRST READING, it's wrong -- and you have backup on that point from the AP Physics Development Committee itself.
(2) "Full credit may not be earned if a paragraph-length response contains...:
* Principles not presented in logical order
* lengthy digressions within an argument
* primarily equations or diagrams with little linking prose."
In other words, it's a paragraph -- use your words, younglings, and stay focused. You can not earn credit by throwing everything that comes to mind at the wall and hoping something sticks.
The College Board has released one paragraph-response question each for AP Physics 1 and 2, in the free response section of the practice exam. I have one more about static equilibrium that I wrote for my tests that I'd be happy for someone to post on PGP-secure -- please, someone email me, I'll send you the file, and you can post it for me. Anyone else have good, vetted paragraph response problems?