The forehand volley is almost universally taught: hand in front of the body, racquet head up and laid back a little (at first), find the ball, and punch forward like a boxer's jab. Step into the ball with the opposite foot is best if you have time. The more one learns to lay the racquet head back and gradually increase the downward path of the "punch" the more backspin will be produced. This will make your return more difficult for your opponent because the faster backspin increases how much the ball "grabs the court" and seems to stop and bounce lower than expected. This down angle will give you more control of the shot, but be sure to drive the racquet into the ball sharply to make the strings bite into the ball or it will just pop up off of your racquet!
Pros are usually in excellent physical shape yet I often see some of them, including "top tenners," blow shots over the baseline and out the side because of the weakness of the aforesaid old method. Although fine for insuring that low balls climb enough to clear the net and give opponent a chance to err, this age-old method ignores the dictum that one can and should take advantage of balls above the level of the top of the net. BALLS ABOVE THE LEVEL OF THE NET CAN AND SHOULD BE STRUCK OFFENSIVELY!
Examples: I watched Serena drive a hard shot crosscourt from the deuce court to Hingis at the 2001 Australian. She had the whole ad court to volley into, but because the biceps' strength wasn't available to control the racquet head orientation, the power of the shot forced her racquet face back a little allowing the "mirror effect" to take over and angle her volley away wider than intended so that it landed just outside of the sideline.
On August 2, 2004, while playing in the Olympics, Mardy Fish, at about 8 ft. from the net and wide in his deuce court, sent a volley down the line fairly deep in Agassi's ad court. Agassi drove a hard two-handed topspin backhand, again down the line, behind Mardy, who had already shifted his weight toward center court. Unfortunately, Mardy had been taught and had been using the old slice volley technique for many years and it was dominant in his kinetic memory. The harder the approaching shot, the more one's control decreases when using the old slice technique. This causes too many unforced errors to be produced because of too little muscular control. Fish was able to get his arm and racquet head angled in front enough to block the ball towards the wide open crosscourt, out of Agassi's reach. However, the weakness in his technique allowed the ball to climb up off of Mardy's string bed and land several feet out beyond the doubles sideline!
Mardy lost the point despite having an open court to hit into! He hadn't had time to execute this second volley with any drive off his back leg. He could only block the shot with the weak "face laid back" orientation. Thus he had no body rotation with shoulder and arm muscles to produce much strength to resist the power and spin of Agassi's shot. His contact point was about 8 feet behind and above the level of the net, so although an offensive topspin shot with bicep power and pronation would have insured a winning volley, his weak "laid back" combination didn't have the strength to negate the topspin Agassi had generated.
I've just watched Taylor Dent blow a forehand volley into the net against Andre Agassi in the crucial 2nd set tiebreaker at the 2005 Australian. Again, coming over the ball may have made a great difference in the match. This young man comes in to volley after almost every serve so adding "come over the ball offensively" would help him much more than does the weak slice method when he is returning balls above net high.
He, Mardy, and the aforementioned Hingis, had only the foreword power of the inside muscles of the upper arm (the Long and Medial Heads of the Triceps Brachii), the Brachialis, and very little of the forearm: mainly the Flexor Carpi Ulnaris, and only the inner edges of the lower two layers of the inner forearm, i.e. the Digitorum Superficialis and the Digitorum Profundus, with some help from the Pronator Teres which extends from the upper to lower arm, helping the elbow flex. The biceps brachii (of the upper arm) can hardly be used at all during this shot other than to raise and maintain the racquet position, so when slicing they can exert no power to the forward thrust. The harder the approaching shot, the more one's control decreases to the point that there is no time for any forward thrust at all.
An experienced player must not try to change their kinetic memory (much too difficult and time consuming). Results will be achieved much quicker if the player attempts to learn a completely new method of hitting when presented with the ball still above net tape height.
Think positively to execute either the blocking shot or the "come over the ball drive volley." The blocking shot has the combined stationary strength of the shoulder muscles (The foreword section of the Deltoid, the Pectoralis Major, Pectoralis Minor, and the arm muscles (Biceps Brachii and all of the locked forearm, wrist and hand) to hold the racquet solidly in place for a strong, well placed block. The strong punch volley is directed by the orientation of the racquet head either at your opponent's feet or it is put-away through an opening for a winner! If your body and legs are in a good position, add their thrust to the punch. You can also add pronation with great effect.
Whether a player uses the Continental (best), Eastern Forehand, or the Western Grip, aggressively attacking balls above net height will produce much stronger shots. But be aware that when the ball is flying back and forth at high speed between net opponents, one doesn't have time to change grips or move the body sideways between forehand and backhand shots. In these situations, only the Continental Grip can be used effectively on both your forehand and backhand sides without a grip change. It can also be used to effectively block back any ball about to hit anywhere on your body or head with the backhand side of the string bed.