> Use random strings at the start of your keys.
  > This seems like a strange idea, but one of the implementation details 
  > of S3 is that Amazon use the object key to determine where a file is physically 
  > placed in S3. So files with the same prefix might end up on the same hard disk 
  > for example. By randomising your key prefixes, you end up with a better distribution 
  > of your object files. (Source: S3 Performance Tips & Tricks)

This is great advice, but just a small conceptual correction. The prefix doesn't control where the file contents will be stored it just controls where the index to that file's contents is stored.