How to reduce the landing roll distance If I want reduce the landing roll distance, for example to clear the runway at the nearest exit, I was told by some instructors that to increase the drag using following methods:


Pull up and hold the elevator at the upper extreme position.


Question 1: does that mean I should try to keep the nose wheel in air as long as possible?Is there any risk I leave the ground again due to aggressive pitch? Will there be any effect after all the wheels touched the ground, like keep the elevator up can increase the friction of the main wheels? 


Retract flap to reduce the lift to create more frictions.


Questions 2: when flap is retracted, I will have less lift so more friction force could be induced but does that mean the aircraft is in a cleaner config, so the drag is also reduced?
1) This does mean you should keep the nose up as long as possible, increasing the angle of attack when you are behind the drag curve increases the drag the wing produces and uses the elevator as a spoiler in effect. If you have done a full stall landing pulling up cannot get you back into the air, but if you land with extra speed pulling up could mean you start flying again, so you should only use this technique if you are sure you have a low enough airspeed. Good technique is not to yank the stick back, you pull it back while paying attention to the "feel" of the airplane, if it seems to want to fly again ease off on the back pressure. 

2) Retracting flap after landing decreases lift and puts more weight on the wheels, making the wheel brakes more effective. It does decrease drag, however your wheel brakes are much more effective at slowing the airplane than drag from the flaps.