If you don't have something that threatens your opponent, then you can't threaten the opponent.  So, how do you develop something that threatens an opponent?

You can't do it by just training everything equally each session. You need to spend a huge amount of time developing the things you can do that can threaten opponents. If you train everything equally, you'll tend to do everything at about the same level, and so will have nothing that threatens opponents unless they are weaker players. 

The first step is identifying what it is you might be able to do that will threaten an opponent. To use a common example, suppose you believe you can develop a very strong forehand. Then you need to spend a lot of time developing 1) your forehand; 2) the footwork to get into position to play the forehand; 3) serves that set up your forehand; 4) receive that sets up your forehand; and 5) rally shots that set up your forehand. 

Once you've identified the aspects of your game that you need to develop in order to develop something that threatens your opponents, the next step is to practice those aspects. And here you may face a problem. If you have limited practice time, you may not be able to get enough time to really develop those aspects. So what do you do? You make time - but perhaps only temporarily. (If you can't find the time to practice, then you simply won't improve.) Perhaps make a deal with yourself that you will put in extra practice time for a set period of time - perhaps six months to a year. Once you've developed those techniques, you have them for a lifetime! (Assuming you keep them tuned up.) 

Here's a tip on practice I've always advised to players: "Practice everything you do in a match, but focus on fixing the weaknesses and developing the overpowering strengths." A sequel to this might be that, if you can turn a weakness into a strength, your game will go up a lot. 

If your opponent is one who analyzes an opponent's game and adjusts to it, and is able to identify patterns and take advantage of it, you need to be unpredictable and vary your tactics. But many players, probably most, do not do this. If your opponent is not one who adjusts, then don't make the mistake of "over-thinking," where you try to mix up tactics that are lost on your opponent. Against the non-adjuster, just use the best tactic in any given situation while not overusing any one serve, shot, or technique so much that he gets too used to it. It's against the smart opponent who can adjust that you must regularly vary your tactics.

Now go look in the mirror. Are you an adjuster or non-adjuster? If the latter, it's time to adjust your thinking!!!  

A major weakness of many players is an inability to change the pace, and thereby throw their opponent's timing off. Not doing so is a quick way of helping your opponent's timing. Many players try to change the pace, but do so unsuccessfully - either because they don't know how to do it, or because they don't really understand the purpose of changing the pace.

Changing the pace doesn't mean hitting a weak shot. It means hitting the ball at a slower pace, but low to the net, with the ball landing shorter on the table. If the opponent is expecting a faster, deeper ball, he'll have trouble adjusting to this.

Against aggressive players, you mostly change the pace to win the point outright via the aggressive player's misses, though be ready to jump on a weak return as well. Against control players, you mostly change the pace to force a weak shot for you to attack.

Also try to find out which side the opponent is most vulnerable to changes of pace. Fast backhand players may have trouble if you suddenly give them a softer return to their backhand, but may jump on a softer ball to the forehand. Big forehand players may time faster balls but have trouble when you suddenly give them a soft one. Experiment, and find what works!

If you are primarily a forehand attacker, many of your opponents will get used to your relentless forehand attacks, often from the backhand corner, especially after your serve. Why not throw them off with a backhand loop? This is especially effective against an opponent who routinely pushes your serve to your backhand corner. Surprise them - sometimes serve and get into a backhand position, and follow with a backhand loop! Too often players only backhand attack when they are caught off guard and they aren't able to use their forehand. Imagine how much more effective this is if you plan it, and have time to prepare. It means you don't have to step around (and, of course, many of us can't do that effectively anyway), you won't be out of position, and perhaps most important, your opponent has to adjust his timing to a different type of loop.

The key is to be ready both to backhand loop if the ball comes to your backhand side, or to forehand loop if the opponent surprises you by going to your forehand. If they do, simply rotate your waist to your forehand side and you're all set. But once in a backhand position, you can, if necessary, cover half or more of the table with your backhand loop.

You may worry that your backhand loop isn't as strong as your forehand loop, but if used as a variation, that's not only not a problem, it's a strength - the contrast will force the opponent to adjust. Not only will the backhand loop throw him off, but it'll make your no-longer-so-predictable forehand loops more effective. One of my best tournament wins ever came when, at 19-all in the third (back when games were to 21, best two out of three), after realizing the opponent had adjusted to my relentless forehand attacking, I serve and softly backhand looped twice in a row - and the opponent missed both.

As an addendum, if you are the reverse - someone who serve and backhand loops against pushes to the backhand - then you should do the reverse as a variation, and sometimes serve, step around, and forehand loop, just to mess up the opponent's timing. Be unpredictable!

In the movie "Searching for Bobby Fischer," there's a famous quote, "Don't move until you see it." It comes from a coach who is telling his chess-playing star not to move until he sees the line of moves he's about to play. Similarly, in table tennis (which we often call "Chess at light speed"), you shouldn't play a point until you have a game plan.

If you don't know the opponent, then the game plan at the start is a mix of forcing your game on him, and figuring out his game. Sometime in that first game you should have a basic game plan worked out. The game plan should be flexible, subject to change as needed. Against an opponent who gets used to or adjusts to what you do, you have to make changes. It is likely different when you are serving than when you are receiving. And the plan changes as the rally proceeds. But at any given time, you should have a basic game plan.

Here's an example of a game plan, one that I might use. On my serve, my basic game plan might be to serve a lot of short backspin, hoping to get a long push that I can loop. If my opponent pushes my serves long, and I am winning by looping those pushes, then my game plan is working. But suppose the opponent's pushes are very good and I have trouble with them, or he starts flipping or pushing my serve back short, and I'm unable to loop. Then I would have to change my game plan. For example, if he's pushing heavy or dropping my serve short, I might serve more no-spin serves, which are harder to load up with backspin or to drop short effectively. If he's flipping the serve, then I'd focus on varying the spin as well as making sure I'm serving very low. (In reality, my game plan would more likely have been to mix in short backspin and no-spin serves, so the opponent has to adjust to both - and I'd soon figure out which one is more effective against this opponent, and favor that one. Plus, I'd mix in long serves so he can't lean over the table waiting for the short serve.)

On the receive, my basic game plan might be to attack deep serves, so the server would be forced to serve short. Against short serves, my game plan might be to go for a consistent flip into the backhand, trying to force a neutral backhand exchange and thereby taking away his serve advantage. But suppose I have trouble flipping his heavy backspin serves, or perhaps he has a very good backhand and so this tactic doesn't work, and if I flip to the forehand, he attacks even better. Then I might decide to instead push his short backspin serves aggressively to his backhand, and see if that works. If he's able to attack them, then I might try dropping them short and low, making it harder for him to attack. Eventually I'll work out which of these is most successful, and favor that receive, though I'd still vary it so he can't get used to any one thing.

What's your game plan? Don't play a point until you have it.