With the continental grip (the gap between your index finger and thumb going over the top of the racket, right?) I find it really difficult to hit flat serves. It seems to me that you have to twist your arm quite sharply to get the racket parallel to the baseline when making contact! Or am I missing something?
I like to toss the ball a little more foward when I hit flat serves I usually let my toss float 3-5 inches into the court. And as always make sure you keep your head still until your follow through is finished. Your head is about 30% of your body's weight so it is important to keep your eye on the ball and not jerk your head around.