It should go without saying, but it's easy to forget. When testing your app, be sure to exercise various combinations of:
- View states: fullscreen landscape, filled, snapped, and fullscreen portrait.
- Screen sizes: from 1366×768 (or smaller, if you have such hardware), to high resolutions like 2560×1440
- Resolution scaling: 100%, 140%, and 180%
You can test these things on actual hardware, of course, which is why Microsoft set up the app labs as I mentioned before.
Otherwise, get to know the features of the Visual Studio simulator as well as the Device options in Blend.
In the simulator, the two rotation buttons on the right side will help test landscape/portrait, and the resolution scaling control lets you choose different screen sizes at 100% and a few that go to 140% and 180%:
In Blend, the Device tab (next to Live DOM), gives you options to show the app in all the view states (the buttons next to View) as well as differnet screen sizes and scalings (these are the same options that the simulator provides):
Make it a habit, then, to regularly run through these different combinations to find layout problems with configurations that some part of your customers will certainly have themselves.