Doing a good job for every client, having reasonable rates, diversifying your offerings for potential clients, followup calls after a session, renting out your studio to producers or other engineers at block booking rates when you are not in session, keeping up with the local band scene and making sure they know you exist by attending their concerts and handing out promo materials, working with non profits at a reduced rate, having good engineering and producing chops, doing parts of projects instead of the whole project (recording vocals, recording drums whatever you are best at) doing voice over work for local ad companies, doing live radio shows from your studio and streaming them on the net for a charge. There are lots of ways to increase you studio income you just have to be creative and listen to what people in your area are looking for. Don't offer services that no one wants or needs. If everyone else is doing hip hop and heavy metal try looking for acoustic performers. Try and be a good resource for the musical community and people will flock to your studio.

Best of luck!