Best Answer - Chosen by Voters
Usually its the toss, but it could be your form too. Get somebody who has had or teaches tennis lessons because they will be able to point out the flaws that you don't know. During a match, my opponent's friend, who was watching our game, told me at the end that everything I was doing on my serve was good, except the toss. He said when I double faulted, my racquet's head was still moving up at the point of contact and thus I was hitting the ball early. I had no idea I was even doing this, so he told me to correct it by throwing the ball further out in front so I wouldn't strike too early. But most likely its the toss, so get somebody to watch and correct it. Also, it might be nerves. I used to get pretty nervous and forget my form.