In Visual Studio projects I almost always use the App_Code folder or a reference to a separate class library to store my most commonly used Methods, like a method that formats dates.
Where and how should I do the same when developing an iPhone App using Xcode?
Is this best practice when it comes to iPhone Apps?
You're talking about common code you want to share between projects, right?
There are a few ways to handle this. Building your own Framework is one possibility, you can see this in action in OmniGroup's open source framework. In my opinion though, building a framework can be pretty cumbersome. I prefer to simply keep a directory of source files that I can reference (not copy) from each project I'm using them in. I don't even do that much anymore in my Cocoa projects though, it seems like most of the truly portable code that I have a use for in multiple projects has been included in successive releases of Cocoa.
See this very similar SO question: Whats the easiest way of implementing own functions that are used all over the place