Basics of Throw-In Technique

To get started with the throw-in, the player picks up the ball using both hands placed at the side of the ball. Typically, a player will use a "W" formation with his hands on the back of the ball. Next, the player must lift the ball above their head, bending their elbows, making sure that the ball goes behind their head, still using both hands on the ball. It is very important to use both hands, otherwise you will not take a legal throw.
Getting Distance on a Throw-In

To throw the ball, the player must then bring both hands over their head, extending the arms. The player should release the ball once the ball is over their head and they must keep both feet firmly planted on the ground.

To make the ball go a little bit further, players can stand off of the line and take some steps towards the line before they throw. If they arch their back and drag their back foot, this will create a greater distance in the throw, but all of the other rules stay in place, two hands on the ball, the ball must start from behind the back of the head, and the player must keep their feet firmly planted on the ground.
Rules Regarding a Throw-In

A throw-in is not a stoppage in play. A player can retrieve the ball and throw the ball in quickly. Also, once a player has thrown the ball in, it is very important for their team that they quickly re-enter the field of play and get back to playing the game.
source
http://video.about.com/worldsoccer/How-to-Do-a-Throw-In-in-Soccer.htm