Limits on the capacity, firing rate, ammunition types and quantity of guns one person can own.
An annual qualification, licensing and registration process, something along the lines of motor vehicles.
Extensive buy back campaigns.
An advertising campaign that targets the gun industry similar to what was done with the tobacco industry.
History of mental illness? No guns for you.
No gun sales at gun shows.
Shut down avenues for selling of guns on secondary market.
I also like what I saw in Europe, where gun owners stored their rifles at a gun club or hunter's lodge, and signed them out for sport.