One of the most basic concepts in sports is that you must practice what you want to do in a match. The corollary to this is that what you do in practice is what you'll tend to do in a match. And yet, when it comes to ball placement, these are two of the most forgotten concepts in table tennis.
It's a basic tactical principle that most of your shots should go either to a wide angle - often outside the corners, to force your opponent to move and cover more ground - or at the opponent's middle, which is usually his playing elbow, the midpoint between his forehand and backhand, where he has to make a quick decision on whether to play a forehand or backhand, and then move into position. (There are also short balls, but that's a separate issue.) And so it's somewhat obvious that you'd want to practice putting balls to wide angles and to the middle, right? And yet, watch most practice drills, and you'll see that in the overwhelming majority of drills, players play to the corners, but no wider, and almost never to the middle. And so they are 1) not practicing what they want to do in a match, and 2) since what they do in practice is what they'll tend to do in a match, guess what they'll tend to do in a match?
So perhaps add two types of drills to your practice regimen. First, instead of drills where (for example) one player loops and other blocks, and it's all corner-to-corner, do this same drill, except both players go as wide as they comfortably can. The looper should try looping the ball outside the corner, often with hooking sidespin to go even wider. The blocker should take the ball quick off the bounce, which gives him the widest angle into the looper's forehand, and so he should block the ball as wide as possible. This doesn't mean going for risky extremes; it means going as wide as you comfortably can go. If you go too wide, you get less and less table, and will lose consistency.
You can do many similar drills. For example, when going backhand to backhand in a drill, why not focus on hitting balls a little bit outside the corners? There is an obvious advantage to going right at the corners, since it gives you more table and so it's safer, so you should find the right balance of when to go to the corner, and when to go for more angle.
Second, do drills where you go to the middle over and over. The most basic way is one player attacks the other's middle, while the other blocks from the middle, either forehand or backhand. The blocker can do this drill either by blocking with just forehand or just backhand over and over, or he can move back into a neutral position after each shot, and then react to the next shot with forehand or backhand. The result of such a drill? The attacker gets in the habit of attacking the middle, and so is able to do so in matches. The blocker practices something he will often see in matches - attacks to his middle - but rarely practices against, and so is rarely prepared for. And so both players improve their middle play.
So try out these drills, or come up with your own that allow you to practice placing the ball at wide angles and to the middle, perhaps by matching patterns you see in matches. And then do in matches what you do in practice!