The most important thing here is to take a close look on the schedule.
Just because Buffon is a better keeper than Frey it doesn't mean that he'll bring you more points than Frey in the particular round where we play Roma away and Fiorentina play at home with Lecce.
Keepers and defenders bring a lot of points for a clean sheet.
Midfielders are also important. Last year Hamsik was real gold and he was the only player i had in my team from round 3 till the end. Midfielders (and all the other players) bring a lot of points for every successful cross, for every corner won and what's really important is to have attackers/midfielders who take a lot of shots on target because every shot on target brings you 3 points.
For example, last year Trezeguet brought less points than Borriello, even though Trez scored more goals than Borriello. It was because Trez scored almost every time he had a shot on target while Borriello needed 3 or 4 shots on target to score a goal.
You'll learn all the tricks as time goes on and this were just a few tips. There are much more things to know and that's what makes this game very complicated, but also very interesting for a serie A freak like myself
P.S: Oh, and the player's value changes all the time. So it's really important to make a correct prediction and buy a player whose value will rise as time goes on.
Last year i bought Hamsik for some 7 or 8 points and at the end his value was around 20.00. Those who bought him later needed to pay 20 points, which left them a lot less money to buy other players.