After sonication you may also try storing your mobile phase in a narrow neck flask containing a stir bar and put the mobile phase on a stir plate at a very low spin. The motion and slight heat from the motor will keep gases from entering you solvents. Seal the opening with aluminum foil, not parafilm.

Sparge your mobile phase with ultra high purity helium gas. This is by far the best way to reduce bubbles, reduce UV absorption of the solvent and reduce the baseline noise. Make sure you sparge continously and do NOT turn the gas 'On' and 'Off' at intervals as this will cause a cyclical baseline noise. *We sparged all of our mobile phases with helium before electronic vacuum degassers became widespread in the industry.