The best cure for begging is to not let the habit kick-in to begin with!
We as parents, control and shape a dog's behavior. If you never want your dog to stick her nose in your plate, put her head on your knee or paw at your arm, then don't ever reward her with food when she does.
When we like the dog's behavior, we say we're "sharing" our food. When we don't like the behavior, we call it "begging." And we foolishly expect our dogs to see the difference.
What if it's too late for that? With patience and consistency, you can change your dog's behavior by never rewarding the begging again. When your dog finally becomes convinced that she will never again see another piece of food delivered from off your plate, she'll stop asking. You can also have her practice a behavior that's incompatible with having her nose on your knee -- a down-stay on the other side of the room while you're eating.
Inconsistency will worsen the problem. Rewarding a behavior occasionally is called random reinforcement, and it's a powerful motivator. In fact, it's what keeps the gambling industry so profitable: You never know when a slot machine will pay off big, but a little payoff now and then keeps you playing.