CAVEAT: I am not your instructor; it would be a very good idea to physically train with an actual instructor.
That said, what I'd advise for basic retention training is this:
1) Do a search for "Blue Gun." Find a blue gun (plastic training weapon) that matches your carry gun, or as close as possible. Ideally, it will fit in your carry holster, so you can practice retaining it in your holster, in its normal position. I do not recommend retention training with real weapons, as people have been known to make very serious (and sometimes fatal) mistakes while attempting same. A blue gun (handgun) will probably cost around $40 online; a blue gun (rifle or shotgun) will probably cost around $160 online. Alternately, your local police supply store may stock or order blue guns.
2) Once you have an appropriate training weapon, and your carry rig, train with a partner. (AGAIN, this would ideally be a trained instructor.)
3) The technique I'd start with, assuming a grab from gun-side rear flank, goes like this:
A) Chop down on the grabber's wrist with the knife edge of your hand; allow your chop to form into a grab or lock as an immediate result of contact. Think of it as forcing your gun into its holster, and your holster into your hip and leg, by applying pressure through the grabber's wrist.
B) As your striking hand locks his into the gun (and most likely your hip), pivot TOWARD the grab. (USE CAUTION, this can really torque the grabber's wrist, as you are using total body mass against his wrist; if we break our partners, we can't continue to practice.) Pivoting could involve rotating on the ball of either foot, though typically the gun-side foot would be the pivot point. The idea is to keep the pivot tight, and make yourself the axis of rotation, while the grabber is forced to the circumference; this may also require you to step or inscribe an arc with a foot as you turn.
C) You still have a free hand, the one that is not locking the grabber's hand. Since you are pivoting toward him, his wrist should be locked, and if you do it right, his elbow will also lock, and his arm will torque his shoulder. This means, if done properly, his body will be forced to rotate away from you, and his other hand will be blocked from striking you by his own body and arm. Since you have a free hand and a free shot at him, punch him in the face, poke him in the eyes, or spear hand him in the throat. (AGAIN, USE CAUTION; if you are not used to controlled sparring, do not come close to eyes or Adam's apples.) Other techniques with the free hand may be more appropriate, depending on how far he has already been off-balanced. We are discussing principles, here, more than specific techniques.
D) While he is trying to not get his wrist snapped, and is being pushed around by his locked-out elbow and arm, and is flinching from your strike to his eyes or throat, use whichever foot is in best position to sweep his foot or collapse his knee. (AGAIN, USE CAUTION; my friends and I train at this stuff, so I know how to collapse a knee from behind, without inflicting injury, but a strike to the side of the knee could do permanent damage.)
E) By this point, if he hasn't released the gun, he probably really wants to. Move away from him quickly, and draw as necessary.
Note that at no point during this drill do I say "Reach behind you and pit your arm strength against his." The whole point is to use mass, movement, the inner radius of a circling move, and basic physiology to give yourself all the mechanical advantage and the grabber none of it. Reaching behind almost always puts you at a disadvantage, and puts your rotator cuff at risk. You have the best, controlled power if you keep your hands in the box formed from your hip points to your armpits, EG like they'd be if you were working on something on your work bench. Turn your body so as to keep your hands and arms in their strong zone.
Your center of mass is a couple fingers breadth below your navel; your legs and butt are your largest muscles. By using your legs and hips to apply torque, you generate maximum power.
You can increase that power by bending your knees as you turn, dropping weight into the equation, along with torque and inertia.
Small people, applying those principles, can overwhelm large strong people. Put it this way, a 95lb woman, applying all her weight and leg strength, might apply well over 100lbs of force to a grabber's wrist. How much do you wrist curl?
But, again, I am not your instructor, and you really should find a trained instructor who can teach you how to do this kind of thing with minimal risk to yourself or your training partners.
Edit: I just noticed that zombietactics alluded to control of the gun, and miketodd04 discussed locking the grabbing hand while going for the face with the free hand. They aren't wrong, but the additional factors I discussed really amp up your leverage while taking away the grabber's balance - which makes locking the hand and attacking the face much easier.
2nd Edit: Before I left home for my current trip to the sandbox, I was discussing the possiblity of conducting a class or two on this stuff with a friend, who is the local prison's deputy warden, and an acquaintance, who is a training officer for the local sheriff's department. If we do this when I get home, I'll see if they are cool with recording some of it, and try to post a link.
