Basically you never touch the collective when the heli is vertical, going up or coming down. Add a little collective just before you pull back on the cyclic- hold the cyclic back- add a little negative pitch once you are completely inverted, not before or after- once the heli is pointing downward at an angle of 45 degrees start adding positive pitch again.
It's actually amazing how little collective pitch you really need to loop, even less for flips- just a little snap when the heli is horizontal.
I think the best skill to learn is collective timing, I don't hold the collective "on", just give it a little punch and return to near mid-stick, small inputs but quick ones-then you don't load the rotor up as much.
Best of luck