Q:

What is the expression for rules that are typically flouted?

I seem to remember that it involves the word "observe", but I can neither recall it nor find it in a thesaurus.

A:

You are probably thinking of more honored in the breach than the observance, which can be found on phrases.org and on wiktionary under "honor in the breach":

(idiomatic) (of a rule, law, or policy) to demonstrate by breaking or breaching  (wiktionary)
This is usually thought to mean a rule which is more often broken than observed. The context of the play [Hamlet] shows the real meaning as 'it is more honorable to breach than to observe'. (phrases.org)

Personally I find phrases.org's "real meaning" to be poorly phrased; it should say something like "the original intent of the phrase was to point out that it is more honorable to break certain rules than to obey them."  The quote is from Hamlet, specifically when Hamlet was observing that it was customary for his stepfather to get all liquored up and engage in shenanigans.  He then says

...But to my mind, though I am native here
And to the manner born, it is a custom
More honour'd in the breach than the observance.

A:

A rule that has little or no practical use and is usually ignored is a dead letter. The other American English meaning, "an unclaimed item of mail," is uncommon in British English. http://www.oxforddictionaries.com/definition/english/dead-letter

