Answer: increasing the pressure of the gas above the solution.
The dependence of the solubility of a gas in a solution with the pressure above it is given by Henry's law:
C = K P, where C is the solubility of the gas in the solvent, K is the proportionallity constant, and P is the partial pressure of the gas above the solution.
So, when P increases C increases, and when P decreases C decreases.