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