The biggest thing of all, I believe, is actually testing your design on a device. Text you think looks good when you're in Photoshop has a tendency to be too small or too large, or too gray or not enough saturation on a device. So, if you're designing - and can't get a build to your phone to actually play with - email yourself an exported PNG of correct resolution and open it up on your phone. You should be able to see and "play" with it as best as you can for absolutely no money.

It seems like this was written for the designer, not developer, so I would also like to add one more thing. If you're a designer and tasked on working with a developer to make an app happen (or vice versa really), then work side by side as much as you can with that person and have regular discussions. You'll both drive each other toward a better project than just piecewise doing things in a more waterfall-esque approach.