Almost all the pros toss roughly "parallel to the baseline" so they can begin to coil back with their upper body as their arm is going up:
Federer Murray Haas & more ball toss common threads http://www.youtube.com/watch?v=lIF-U...B1814186A2E18C
But there is an important point that almost all the pros do to get the ball to go out into the court for the first serve or a slice serve - and Christophe explains this starting at 2:45 in this video:
Secrets of the Toss http://www.youtube.com/watch?feature...R_NaTvPo4&NR=1
Basically, almost everyone starts their serve motion like this:
But as they rock back, they bring the ball all the way back to a point near their BACK LEG !!!
... so that as the arm goes up it moves forward into the court just slightly.
Below, Pete Sampras shows how to toss for a first or slice serve to get the ball to go out into the court:
In pic 1, he has already "rocked back", bringing the ball to near his BACK leg (note the ball overlying the side of his left foot).
By pic 2, with his arm halfway up, the ball is now at a position midway between both legs.
By the time of pic 4, he is releasing the ball at a point over his FRONT leg/foot.
What you DON'T want to do is what the player below is doing - radically changing the path of his arm, and not starting to coil at all during his tossing motion [even though the position of the ball is where you will want to toss it to for contact]:
Instead, do what I recommend to get the ball out into the court for a flat,
topspin-slice and pure slice serve.
The only difference is that for a flat serve get the ball all the way back near your back leg, and for a pure slice start with the ball more between your legs so that your arm will not sweep forward into the court.
As LeeD has written above, once you have the basic idea and motion down of what you need to do, it is not that complicated.
Almost by magic, if you visualize where you want the ball to go, you will start to toss it there.
But of course if takes a lot of practice to do it precisely time after time - at the same time you are rotating back away from the ball as you coil AND toss at the same time.