Recently I was re-routed involuntarily by American when bad weather cancelled their inbound flight from ORD to PHL which therefore cancelled my outbound flight from PHL to ORD. American did an excellent job switching me to America West so that I could make a friend's wedding in San Francisco the next day. When I wrote to AAdvantage explaining the situation, I included my e-ticket stub and AA/AW boarding passes. Within two weeks, my account was properly credited for the mileage as well as the Gold bonus miles.

On rare occasions when credit didn't flow so smoothly, I have kept one half of the original receipt by tearing along the perforation (and half the boarding passes) which allowed me backup "original documentation". It really comes in handy when you need it for a second letter!

By the way, American will not issue a voucher when weather cancels your flight. They leave any voucher decision to the sole discretion of the agent who originally handles the problem. By contrast, TWA sent me a voucher once when a problematic onboard drunk was not handled to my satisfaction. Southwest and Continental have given me vouchers on the spot in the event of flight cancellation regardless of cause.

In fairness, American's customer service department gave me double miles on the cancelled portion of my trip as an apology (which was better than nothing).

So keep your original documentation (or at least copies), but also write to the general customer service department, not just the frequent flyer division.