Servos were trembling, especially when reaching 100% PWM duty cycle

[retrolefty]

You don't 'talk' to servos with analogWrite() commands. You include the servo library and use servo write commands as in the
following example.

#include <Servo.h> 

Servo myservo;

void setup() 
{ 
  myservo.attach(9);
  myservo.write(90);  // set servo to mid-point
} 

void loop() {}

More info can be found here: http://arduino.cc/en/Reference/Servo