It goes against every natural instinct I have to completely ignore when my son hits his sibling or another child, but nothing I tried worked until I tried this. And I have Ralphie Jacobs of Simply on Purpose to thank (seriously, if you have kids of any age you need to follow her!).
The basic philosophy that Ralphie teaches is that you shouldn't be giving attention to inconsequential negative behaviours, or behaviours that are mostly attention seeking and not intended to do real damage. Her mantra is, "water the flowers, not the weeds." Often, hitting is an attention seeking weed rather than a deliberate attempt to cause harm, so giving attention to it just makes it grow.
Author Catherine Pearlman backs this idea up (she even wrote an entire book titled, Ignore It!). "It may sound counterintuitive, but sometimes ignoring your child is the perfect response. Any reaction to these annoying behaviours will merely serve as a negative reward, which actually encourages the behaviour to continue."