Where should you contact the ball when serving? This is one of those subtle things that many players spend their entire playing lives or careers never realizing they are giving their opponents an advantage. Where you contact the ball makes a significant difference on how your serve goes out and how it is returned. Here are three things to consider when serving.
First, your contact point should be low. The higher you contact the ball, the higher it'll tend to bounce on the other side. At minimum, try to contact the ball no more than nine inches or so high, but ideally even lower, even lower than the six-inch net. The lower you contact the ball, the easier it is to keep the ball low. And keeping the ball low when serving is one of the most under-rated parts of serving. It's not just that slightly high serves are easier to attack - they are - but also that super-low serves have to be lifted over the net, forcing weaker and less consistent returns. They also cut off most aggressively angled returns.
Second, contact should be as close to the table's end line as possible. If you contact the ball a foot behind the end-line, you give your opponent that much extra time to react to your serve. Why give him that time? Some players do toss the ball backwards when serving, since this allows them to essentially throw the ball back into their racket, which can give extra spin. (It also makes it easier to illegally hide the serve.) The rules state that the ball must be thrown up "near vertical," so there is some leeway here. In general, however, you want to give the opponent as little time to react as possible, so even if you throw the ball back some (within the confines of "near vertical"), you should balance this against the extra time the opponent has to read your serve.
Third, for fast serves that go deep on the table, move the contact point back. To maximize the speed on your serve, you want the ball to travel over the table for the maximum time between bounces so that gravity (and topspin, if you served with that) has the most time to pull the ball down, thereby maximizing how fast you can potentially serve. This means the first bounce should be as close to your end-line as possible, with the second bounce very deep on the opponent's side. To do this, you need to contact the ball a little behind the end-line, perhaps a foot back. The contact point should be very low to the table, no more than a few inches higher than table height, allowing you to serve the ball mostly forward and very low to the net, to maximize the speed you can put on the ball and still keep the ball on the table.
So focus on the position and height of the contact point on your serves, and take your service game to new heights!