I OC exclusively, without a retention holster. Here are a few pointers (I don't pretend to be an expert, and this should only be considered advice, not instruction):
1. Always be aware of who is around you, be hyper aware of anyone in your safety zone (grocery store checkout line, crowded store), especially if they do not belong there or are acting suspiciously (pre attack indicators).
2. Always keep your gun arm over your sidearm (supposing you carry at 3:00), either hand in pocket or elbow over grip. Scan the area before reaching away from your sidearm or other activity that would expose the sidearm to a gun grab.
3. Act normally, smile, greet and make polite conversation as appropriate to make people at ease, and let any potential BGs know you are aware of them.
4. Keep moving as much as possible to avoid static environments where a BG can stage on you or work up a plan for a gun grab.
5. When you must be still, position yourself so that your sidearm is away from other people, such as against a wall, shopping cart, display stand, or one of your party.
6. If you are the victim of a gun grab, there are options. Most holsters use a snug fit to prevent unholstering. Body movement is key to prevent the draw. Place your hand/elbow/arm down hard over the BG's hand to prevent the draw. Twist your body in a way that will allow you to strike/knife/pepper spray your opponent. Fight for your life. If your gun is taken, expect to be shot.
7. Learn the methods of disarming your opponent and practice them. Videos are available on Youtube. There are classes also.