Many don't realise that designers and developers are usually two different things, the same applies to app designers and app developers. They both play their own roles and usually they're not great at doing both. If you find someone who can do both, hire him/her...now.
This sweet little infographic is a concise guide for iOS app designers. Obviously some of the tips in the guide below don't just apply to iOS designers, they can also apply to other platforms like Android and Windows Phone.