The first thing to address is the friction of the ball links on all linkages. The links should snap onto the balls but not be so tight that the link and its control rod can't swivel under its own weight. Disassemble the links from the head, swash plate, servos, and tail. Working one at a time, connect each connecting rod's link to its associated ball and make sure that it moves freely in its normal operating position. The link and its connecting rod should swing like a pendulum. If you have to push the connecting rod to move it, the ball link has too much friction and needs to be loosened.
Loosen the ball link by squeezing it with a pair of pliers (smooth jaws work best) as shown in the photo below. You may need to squeeze the ball link multiple times to reduce its friction.
Addendum by Bert Van Kets
When using regular plyers you risk snapping the links when the plyer slides off. By using a Vise Grip plyer you can set the minimum opening and adjust if necessary. This way you don't risk of the plyers snapping shut, taking the link with it. I use the smallest needle nose Vise Grip plyer with great succes.
Note
It is important not just to squeeze the sides, but to try and squeeze all around the ball links. You can't do the top as the link itself is on the way, but the rest of the “ring” can be loosened up pretty well
