When you've completed these first two steps, the next thing to do is to spray down your mattress with a special little concoction: water, plus a few drops of an essential oil (such as eucalyptus, peppermint, or lavender). This will disinfect it and protect it from further bacteria buildup.
And although flipping the mattress is not enough alone, it's still recommended that you do this from time to time, even if it's only every 3 or 4 months.