First off i would like to say that User experience and functionality of the app is equally or more important than it's UI. There's no point in a app that has a awesome looking UI but is very laggy or lacks functionality.
Lets begin with a few tips :
- Try to make the UI simple and clean with no extra clutter.Hide extra functionality in menus. Keep the main function of the program in the front.
- Make the UI flat.Use subtle gradients and shadows. Don't make it too flat like the metro ui,in which you can't distinguish an icon from a button
- Use a specific color scheme. Select two to three lead colours. and select 1 or 2 background colours.
- Either make it dark themed, or light themed, don't mix the two!
- Use transitions. But not too fancy.
- Split your app into different parts and provide a easy way to navigate
- Fix Whatever you can behind the scenes
- Use short Phrases, No one wants to read too much to do a simple task!
- Use pastel colours or darker colours. ie. one shade lighter or darker than the original color.
- Have contrast between the text color and the background color, making it easier to read.
- Don't waste much time making animations. Use LayoutTransitions (android 3.0+)
- Test your apps on different screen sizes.
- Download the color swatches provided by google -click here
- Google is your friend. Go to developer.android.com for more guidelines, application fundamentals etc.
- Use good typography. Roboto and helvetica are some of the good fonts. Download roboto - click here
- Guide on iconography - click here
- Do not use harsh gradients! for eg.. black to white , It makes the app feel cheap.
- Try and stay away from putting ads
- Do not let the app make decisions for itself, let it ask the user!
- Do not make the app too colourful.
- Make sure it runs smooth without jitter.
- Don't put too much info. at once!
- Don't use too flashy colors that hurt the eye.
Again, this isn't meant to be a very long guide, just a few basic things you should keep in mind while designing. For more detailed guidelines use developer.android.com
If i missed something important let me know!