How do you increase the gravitational force between two objects?

Either increase the mass, or decrease the distance.

When two objects get closer to each other increases the gravitational force between them increases.

the gravitational force will decrease

The magnituide of the gravitational force between two objects will increase if -- the mass of one or both objects increases OR -- the distance between their centers-of-mass decreases.

If we have to increase gravitational force between a chair and the earth, put chair close to center of the earth. As a reference for gravitation force it is always measured from center of two...