if you are talking about the throw into the line you always see ppl throwing at the posts but I find if you only have a couple of balls you want something that the ball is going to bounce off so you dont have to keep retreiving them.
At school in your breaks go into the gym and throw at the backboard of the basket ball net. On the throw in break it down into parts and concentrate on one point over and over before the next. When i was learning I thought to my self right how an I going to hold the ball, personal preference I supose, steve Thompson holds it with both hands towards the back of the ball, others use their strong hand at the back and the other at the front with a light grip only as a guide. Ibanez obviously only uses one hand (wouldnt recomend this!).
after sorting that out I started concentrating on bending at the hips and putting more of my body into the throw. Finaly the follow through is important, I like to excentuate point my hands where I want the ball to go to and stretching right through the whole of my body making sure that your head point down and I like to finish with my hands up high not down by my hips as I find that if my head falls or my hands finnish down there thats where the ball will go.
then once you have practiced with specific attention to one aspect them go on to putting it all together. I also always aim higher than I actualy want the ball to be.