Normally when I am asked to write a reference for someone who was poor I simply state:

  <<Name>> worked at X from <<date>> to <<date>>. His/Her 
  role was that of <<position>> and had responsibility for 
  <<responsibilities>>. He/she led a team of <<number>>.

If the person was good, I'll state it explicitly. I expect the new potential employer to read from what I didn't say when it comes to someone that was not particularly good.

Does anyone else do the same?