Practice handstands, as this move incorporates many of the muscles required for break dancing balance. Keep your shoulders level with your hands. If your shoulders go in front of your hands, you are not in a balanced alignment. Start by learning with a wall, then move to free-standing handstands. Handstand pushups also help build the core and upper body strength required for difficult balance moves.
Train your arms -- especially forearms -- with weights in addition to your dance training. Moves such as stabs require a great deal of weight to be supported by the forearm and wrist, so strong muscles are a requirement for balance.
Work on a basic freeze with a stab. Put one arm lower than the other, with the elbow just inside the hip. Outstretch the other for balance. Use your head for more balance and lift up your foot on the side of your balance arm. Rest it on the knee of the other leg. Keep the vast majority of the weight on the hand near the hip, but use the other points for balance. Practice this until you can maintain perfect stability in the position.
Practice transitioning in and out of the basic freeze. Work on it to the point where the balance becomes automatic and fluid. It shouldn't be difficult to get into the freeze and hold the position.
Vary the movements. Work on freezes that provide you with less balance points and put you further out of balance. Practice transitioning in and out of the moves from a variety of steps. Incorporate freezes during all your practice to continually improve your balance.