Before you serve you should always visualize what the ball is going to do. It amazes me how many players just serve without really doing this. Top players have practiced their serves so much that this is instinctive - they don't think about it, they just know exactly what the ball is going to do. Visualizing a serve means seeing in your head before you serve the contact, direction, speed, spin, height, depth, bounces, and curve of the ball.
One fun way of practicing this is to make your serves do tricks. For example, try serving backspin where you graze the ball so finely that all your energy goes into backspin - and so the serve bounces backward into the net. But it's not enough for the ball to come backwards - you should be able to visualize its actual path in advance. Try serving where you visualize how deep the ball will go and how many bounces before it comes backwards, and the direction it'll come back (since most backspin serves have some sidespin as well). This visualizing includes what the ball does on your side of the net - how fast the ball will travel, where the first bounce on your side will be, and how low to the net it'll be. (When first practicing heavy backspin serves or trying to make the ball bounce backwards, don't worry about serving too low. But as you master the serve, you want the ball to practically skim the net.)
Serving backspin so that the ball bounces back into the net isn't really a serve you need in a match. In fact, it's better to drive such a backspin serve out more so that the second bounce is near the end-line, making it difficult for the receiver to attack, push short, or rush you with a quick, deep push. But the key is being able to control the serve - and you can't control it unless you know what you want the ball to do. Try to visualize the entire path of the ball in advance, including both bounces on the far side, and the way the ball curves between bounces if there's sidespin.
Do the same with a regular sidespin serve, where you serve to one side of the table, but curve it back to the other side - but visualize the curving path of the ball in advance. Perhaps set up a target and try to curve the ball into the target.
Then do the same thing with your deep serves, perhaps putting up targets on the far end-line, and try to hit them - again, visualizing the entire path of the ball in advance, right up until it smacks the target. Then do the same with all your other serves, with or without targets.
At first, this visualizing will feel like a hassle that slows you down. But soon it'll become second nature, and you won't even think about it - but you'll have master control over your serves.