1. Change the surface materials: Different materials have different coefficients of friction. Rougher surfaces generally have higher coefficients of friction, resulting in greater frictional force. For example, rubber on concrete has a higher coefficient of friction than wood on ice, leading to a stronger frictional force.

2. Change the normal force: Frictional force is directly proportional to the normal force. The normal force is the force that presses the two surfaces together. Increasing the normal force (e.g., by adding more weight on top of an object) will increase the frictional force. Conversely, reducing the normal force (e.g., by lifting some of the weight) will decrease the frictional force.