#1: Do not let Barcelona run
Don't let anyone fool you. Barcelona use the opposition's failed counterattacks to score a lot of goals in transition offense. Is far more comfortable for them -and for any team- to score that way than to do it against tight defensive lines. Therefore, Real Madrid's midfielders will need to be smart and prevent players like Iniesta, Neymar, Pedro or Messi to find open space behind the defense like in this goal. That's a brilliant pass by Messi that should have never happened. Bad defending and bad pressure.
#2: Illarra, help the centerbacks against Messi. Khedira, help Illarra.
Xabi Alonso was a great defensive asset for Real Madrid's centerbacks against Messi. But he will not be available for this Clásico, so Asier Illarramendi will be the one in charge of Real's defensive midfield. He's been playing well but he lacks Alonso's experience, so he will surely need Sami Khedira's help.
#3: Wingers, help the midfielders and fullbacks
Ancelotti will reportedly play this match with Di María and Bale on the wings. Luckily enough for Real Madrid, both players have a good workrate and they will be able to build a 4-5-1 when los blancos don't have the ball. As you can see, this is a game of defensive helps. If Khedira helps Illarramendi, Real's right winger will need to help Arbeloa against Iniesta-Neymar. If Messi is covered but Neymar or Iniesta are open, Barcelona will be equally dangerous. Real Madrid need that 4-5-1 without the ball.
#4: Run, Marcelo, run
We know you're a world class player, Marcelo. You're probably the best offensive fullback in the world. But we need you to be a little bit more defensive-minded against Barcelona. Real Madrid will need the Brazilian left back to contribute both offensive and defensively. Marcelo's unpredictability could help a lot when Real Madrid have the ball, but he will need to track back in order for Real Madrid to accomplish #1.
#5: Exploit Barcelona's lack of defensive pace
Cristiano Ronaldo. Gareth Bale. Ángel Di María. Through balls. Enough said.