Letters of recommendation that are "form letters" are quite often recognized as such.
List some of the person's best qualities, provide a couple of short anecdotes that show how that person performs well under pressure/ is able to accomplish huge things/ is an outstanding employee/ etc.
That is, unless you're really well known and respected in the field. In that case, a simple - "I am very familiar with so-and-so's ability and work habits. Without a doubt, I recommend him for..."
I'm familiar with a college case where an admissions counselor received a glowing letter of recommendation for one of the students applying for admission. Several days later, she received a glowing recommendation for another student... and recognized that recommendation. There ended up being three identical letters of recommendation from the same teacher (or principal; that I don't remember) - only the names were changed. That wasn't held against the applicants - they had no control over what was written. Of course, they also didn't really have anything positive said that would help them if they were in a borderline situation. But, the important thing to note is that the person who wrote those letters lost all credibility.