Pushing down works well with most types of holsters and is the only way to go with an IWB.
With an OWB holster, try grabbing the bottom of the holster and pulling it up and out. This will lock the butt of the pistol into your waist area and is very effective with a holster that is securely attached to the belt. This technique, for obvious reasons, will not work with a paddle holster.
For an attempted snatch from the rear, quickly turning towards the side you wear your pistol on will often break the attackers grip. From the front, which way you turn will depend upon which hand the attacker uses to grab your weapon. Movement is simultaneous with other defensive measures.
Since movement and defensive measures must be instinctive to be effective, get with a buddy or SO and practice at half speed to prevent injuries. Ring’s Blue Guns are highly recommended training tools for retention drills. www.blueguns.com