Most of the time you should be watching the ball, but not always. If the ball is moving slowly, you should watch it all the way into your racket. If the ball is moving fast, you can't react at the last second, so there's no point in watching the ball right to contact. In both cases, as soon as watching the ball no longer is beneficial, you should be looking up to watch your opponent. You should generally be aware of the opponent and what he's doing and where with peripheral vision, but actually looking up to watch him is even better. That allows you to quickly prepare for whatever he's doing. You should pick up the ball again just before he hits it, and then watch the ball the rest of the way - until it's time to look up again to see your opponent.

The Chinese know something about table tennis, and there's a stereotype about Chinese coaches that we can learn from. The stereotype is that in a group training session, they often walk around with their heads down as they do two things - listen to the sound of contact and watch your feet. 

They listen to the sound of contact because a good stroke and good timing result in a "good" contact sound. If the sound is wrong, then something else is wrong. By listening to the sound, they can identify there's a problem, and then look to see what the problem is. (So then they do have to look up. You don't want them to look up.)

They watch your feet because the feet are the base of everything else. There's a reason nearly all table tennis drills are footwork drills - many coaches will say all table tennis drills are footwork drills. Get the feet right, and the rest follows somewhat naturally. Get the feet wrong, and all is lost.

So get the sound and feet right, and you won't have to face the fury of all the problems they cause when done improperly. (I'd add one thing to this - if you get both the feet and grip right, often everything in between falls into place. Get either wrong, and everything gets twisted in between.)

Most players practice their forehands and backhands with a 1-2 stroke: backswing, forward swing. But think about it - how often would you do this in a game? Answer - never! In a game, after finishing a stroke, you would return to a neutral position, preparing for the next shot, since you don't know if it'll be forehand or backhand, or even what type of stroke it will be. So why would one practice doing a backswing immediately after finishing the forward swing part of a stroke?

Instead, practice using a 1-2-3 stroke: backswing, forward swing, return to ready position. This is what you do in a game, and so this is what you should practice. There's also a little nuance here in that with the faulty 1-2 stroke, you backswing directly from a forward-swing position, when in reality the backswing should start from the neutral position - so practicing this wrong leads to bad technique and poor timing in games where you have to do it differently. 

Do you think you have a better chance of winning if you focus on winning, thereby putting pressure on yourself for results, or by focusing on playing well, which maximizes your chances of actually winning? The question answers itself.

You'll always play better and have a better end result if you focus on performance (i.e. playing well) rather than the end result. Performance in this case means all aspects of the game, including strokes, footwork, serve & receive, tactics, and the mental game.

So how do you maximize performance, thereby maximizing your results? By practicing your techniques until they are second-nature, even in a big pressure match, and then approaching those matches where you don't increase the pressure by pressuring yourself to have good results.

In other words, try to have fun when you play so there's less pressure, since it's that self-made pressure that causes one's game to fall apart. Practice your techniques, focus on performing well, have fun, and the results will eventually take care of themselves.

Having fun doesn't mean laughing out loud. Have fun on the inside from the satisfaction of working hard and doing what you have practiced. If you focus on winning, you'll just put pressure on yourself and get nervous. Focus on performance, and smile on the inside.  

The above should be on a banner at every playing hall in the world. Players almost always try to drill at speeds or with power they can't control, thinking that by drilling that way, they'll learn to play at that pace. Superficially, it makes sense. But in reality, trying to play at a pace you can't control leads to sloppy, rushed technique, and poor balance, and so you are just reinforcing bad habits.

Instead, drill at a pace that you can control, both with consistency and where you can keep the ball roughly where you are aiming, while staying balanced throughout. Focus on developing good technique, which is the difficult part to master. By going at a slower pace, you can reinforce and perfect that good technique until you can practically do it in your sleep, which should be your focus. Because if you develop good technique, you will be able to play with speed and power - see the title of this article.

What you can and should do, when you are practicing at a pace you can control with at least decent technique and balance, is occasionally smash or rip a loop, just to test the shot. You'll find that with good technique, this is easy, as long as it isn't forced, i.e. you don't try to rush it when you are not ready for the shot. You can also work on playing at a faster pace in multiball or on a robot, where every ball comes out the same, and so you can increase your speed and power and still have control and consistency.

Where did I learn this lesson? It comes from many decades of coaching experience, but it originally came from a specific incident back in my first year of play. At some big tournament I saw U.S. Men #1 Danny Seemiller (soon to be 5-time U.S. Men's Singles Champion) warming up by doing simple side-to-side forehand footwork at a nice, consistent pace with his practice partner and brother, Ricky Seemiller. I remember thinking to myself, "I can do that faster than he's doing it, and he's the best in the country?"

Then I practiced it with someone, and of course I did do it faster than Danny - except I would hit maybe three raggedly rushed shots and miss, my shots were spraying all over the table to my partner's chagrin, and we couldn't have a good rally. Then I slowed down to a pace about the same as Danny and Ricky were doing, and suddenly I was consistent - everything came together, and my shots were fluid and consistent. I was hitting like Danny Seemiller!

From there on I always did footwork and other drills only at a pace I could do consistently and comfortably, with good technique. This doesn't mean you don't push yourself, it means you push yourself at a pace you can do consistently.