A couple of things that have bitten me lately:

• @selector(apply:) is different than @selector(apply)! Note the lack of a colon in the second one. The colon means that there is one parameter to be passed with the message. No colon, no parameter. This makes a big difference when passing a selector as an argument to something like NSArray makeObjectsPerformSelector:.
• I keep forgetting to pass nil as the terminating argument to methods like NSArray initWithObjects:. Forgetting the terminating nil will cause the iPhone to crash with a Bad Access exception as it runs off the end of the stack into the weeds.

And one very valuable debugging tip I found here:
how to find out what exception was thrown in the Xcode debugger. When an uncaught exception kills your iPhone app, Xcode doesn’t really have much information at that point. To remedy that, go to Run -> Show -> Breakpoints and in the Breakpoints window double-click on the entry that says “Double-Click for Symbol.” Enter “objc_exception_throw” and press Return. Now, when you run your app in the debugger, any exception will stop there and let you see the exception type.