A while back I referenced "Twenty Rules for Writing Detective Stories" as a great tool for mystery writers to use when working on their manuscript. It is a great tool and offers advice like "the reader should have the same opportunity as the detective to solve the crime," and "the villain has to be someone who plays a prominent part of the story" (no sudden introductions of the killer at the end of the book). The list of rules reminds us that the killer shouldn't be a professional crook, or a servant (none of "the butler did it.")
One of my favorite parts of the mystery writing rules states:
20) All of the following tricks and devices are verboten. They've been done to death or are otherwise unfair.
a) Comparing a cigarette butt with the suspect's cigarette.
b) Using a séance to frighten the culprit into revealing himself.
c) Using phony fingerprints.
d) Using a dummy figure to establish a false alibi.
e) Learning that the culprit was familiar because the dog didn't bark.
f) Having "the twin" do it.
g) Using knock-out drops.
h) If the murder is in a locked room, it has to be done before the police have actually broken in.
i) Using a word-association test for guilt.
j) Having the solution in a coded message that takes the detective until the end of book to figure out.
Good reminders.
As great as it is, though, this list of rules was compiled in 1936 by author S.S. Van Dine and includes some possibly outdated advice, too....like "the detective should not have a love interest." I don't see a lot of problem with the detective having a romantic interest (poor guy or gal has to have some fun in the book.)
A different version of a mystery writing rules list can be found here. It's an About.com article on modern mystery rules. It does repeat some of the items on the earlier list, but also adds things like: "The culprit must be capable of committing the crime," and "wait as long as possible to reveal the culprit." Their reminder to make sure to research your details is also a good one.