If your android targetSdkVersion is 12 or less, add the android:configChanges="orientation|keyboardHidden"
property to your android manifest.
If your android targetSdkVersion is 13 or more, use orientation|keyboardHidden|screenSize
as the value for android:configChanges
.
Overall it should look like this:
<activity android:name=".MyActivity" android:configChanges="<your value here>">