1. Check Popular Apps
Before you actually start writing your own apps, you definitely want to research the market if there are already exisiting apps providing the planned features. If there are, then take notes about what they provide and ask yourself if you are confident in doing better.
2. Model After Existing Sample Codes
If you installed the Android SDK, there are several great sample projects inside sdk/extras/android/support/samples showcasing almost all the user interface examples and major functionalities, which you can use as templates to build and modify for your apps.
3. Avoid Undocumented APIs
Some code worked at first but after a few Android SDK updates, it stopped working.
4. Layout for Device Orientation and Various Screen Sizes
Since devices have embedded orientation sensors and they are usually defaulted to automatic mode, we need to account for that to provide a better user experience.