It is very easy rule in this you only have to remember one thing that in series branch voltage must be divided where as in parallel branch it remains same.
Explanation:
It is easy in a sense that you only have to remember that in parallel branch voltage always remains same but in series branch it divides across different elements.It can be remembered by a simple thinking that in cars or any other place where a battery requires it will be applied across terminals of a device so that full voltage can be applied on that device.From this simple example you can remember in parallel voltage remains same.
As it is shown by a simple and clear example.
Here E is the applied voltage source across a simple linear circuit which consists