Brent Simmons has a fantastic reponse post to my earlier deadly Objective-C sins article. In it, he lists quite a few ones, and my favourites involve the static analylzer and compiler warnings: treat warnings as errors and turn on all the warnings.
You're code just got, like, instantly more stable. Once you fix all the warnings, that is.
It's a fantastic list — anyone looking to improve their Objective-C chops this year would do well to read it.