Maicun, I was having the same trouble, but I couldn't even get an oval. My pizzas generally looked like random European countries. France or Spain if I was lucky, but sometimes England or Italy.
The pizzamaking.com tip that saved me was "start round to end round". About two hours before I bake my pizza, I take the dough out of the fridge and put it in a round cerial bowl that has a thin coating of oil. I let it sit in this bowl till I'm ready to shape it. Then I press it out, taking care to maintain that circle. Once it gets out to about 10" or so, I can pick it up and stretch it by hand.