App icon guidelines
App icon is an important part of your app. It becomes a symbolic identity for your app. It should be attractive enough to drive the app downloads in the App Store and should stand among other apps on your customer's devices.
You should put in extra efforts to make a good icon and test it on different screens before releasing the app. Tools like IconTester.com let's you test your app icon on iOS and Android devices. Also it prototypes your app icon on different screens of Apple's App Store and Google Play Store. So it's a good idea to test your icon before releasing your app.
Use bright colors
Humans are more attracted towards bright color. In latest iOS 8 and Android Lollipop operating systems you'll notice flat icons with attractive colors.
Don't try to add too much elements or detailing in your app icon. It should be simple and convey the purpose of your app. While making an app icon on computer screen you might find it attractive but as a rule of thumb, you should test it on multiple devices and app store screens using tools like IconTester.com.
Convey Overall app idea
Your icon should convey an overall idea of your app. Failing to do so might confuse the customer and they won't download the app from App Store or Google Play.
Apple doesn't allow transparencies. Create a square shaped icon and Apple will automatically convert it to rounded square. Always test your icon to make sure that the main elements are not being trimmed.
Android allows transparency in app icons and suggests outlining on main elements. This makes the icon prominent on home screens.
You should avoid using texts in your app icon. It's difficult to read the text in icons and it wastes the real estate of your icon. Instead you can put images that conveys the message.