I recommend using Auto Layout if you are not using it yet and your target platform supports it. Auto Layout is a subsystem that automatically rearranges your views when the screen frame changes, according to a set of constraints that you specify, either programmatically or in your storyboard.
Here's the official documentation: https://developer.apple.com/library/mac/documentation/userexperience/conceptual/AutolayoutPG/Articles/Introduction.html
However, there will still be some screens that you should design independently for iPhone and iPad. That's because Apple design guidelines encourages the use of different designs for iPhone and iPad (ie. an iPad app is not simply an enlarged iPhone app). You will probably need to use different controllers to present the model (a UIPopoverController
o a UISplitViewController
in iPad, for example). I suggest spending some time thinking about how your iPad app will behave. Read the documentation to know about the view controllers that are unique to the iPad.
And finally, here's some guidance from Apple about creating universal apps: https://developer.apple.com/library/ios/DOCUMENTATION/iPhone/conceptual/iPhoneOSProgrammingGuide/AdvancedAppTricks/AdvancedAppTricks.html