Question

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

Answer 1

The frictional force between two surfaces is given by:
$F_f = \mu F_p$
where
$\mu$ is the coefficienct of friction
$F_p$ 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 $\mu$ (for instance, by making the surfaces more "rough"
2) by increasing $F_p$, 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 $F_p$