So the short answer is to go slow, do a little, check for damage, then repeat. For the curve I was going for, I would only apply pressure directly below where the pliers were holding the part. I pinched the pliers on the part every 2 centimetres or so down the length of the nacelle. There will always be some spring-back. So when I glued the two halves together, I used clamps to hold the shape while the glue set. For one or two sections I had to use my hands as the clamps kept slipping off the curved surface.
Now if cracks appear, but the parts are still together, all is not lost. That's what putty is for! Its like drywalling a room, don't worry about getting everything perfect, you hide the mistakes in the finishing! Like drywall, its the puttying and sanding that does the trick!
