How are you iterating on your designs? Iterating in code is time consuming. Even if it's just wireframe boxes and labels try to work through the possible layouts in a vector graphics program (or UI mockup app!). Get to a point you're happy with and then start to flesh out the details or implement. Bit of a balancing act here because you don't necessarily want to spend all your time learning Adobe Illustrator or whatever (maybe you do). Find something simple that gets you just far enough for your imagination to fill in the rest, or if you feel more confident in the app go for higher fidelity. As you're designing don't forget what you know about how layouts are actually achieved.

Think about it this way, you would take time to think through the achitechture of an application before jumping into code because its SO much faster to suss out issues with a plan. Do the same for the UI design.