Here's another way to do math in your head! This is my 6 year old's preferred method. She even came up with it on her own! Look through all my mental math videos, try out all the methods, and decide what your favorite is!
If you are teaching mental math to children, give them many problems of one type until they become proficient in that particular way of mental math. Then show them another way until they can do it in their heads - without manipulatives. Keep doing this until you have shown them all the methods or until they are quick enough with one method that they don't need others. Let them choose which method they prefer. None is better than any other.
Most children can be taught mental math strategies once they have their facts to 10 memorized - somewhere between Kindergarten and 2nd grade. Mental math is an excellent way to build and ensure number sense!