For a great description of how the DNA evidence that's turned up over the years has revealed the shoddy nature of our legal system, check out this excellent article by Ed Brayton. It's as far from a scientific enterprise as we could get, particularly when it comes to eyewitness IDs. One commenter did have a noteworthy recommendation:
"I was able to give a very good description of the assailant, even though I said I couldn't ID him. Rather than concentrate on his face, I concentrated on getting a good look at his clothes - particularly the pants and shoes (they're harder to change), and comparing his height and weight to mine. It actually ended up being better information to ID the eventual suspect, who was caught and convicted."