How can you increase the magnitude of friction between two surfaces ? how can you decrease the magnitude of friction

The frictional force between two surfaces is given by:

where
is the coefficienct of friction
is the force with which one surface is pushed against the other one

Looking at the formula, we see that there are two possible ways to increase the friction:
1) by increasing the coefficient of friction  (for instance, by making the surfaces more "rough"
2) by increasing , the force  with which one surface is pushed against the other

Similarly, in order to decrease the friction, the two possible ways are:
1) by reducing the coefficient of friction
2) by reducing the force 

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