Google 'annealing photo etched parts,' or do similar searches on modeling websites...like this one.
The process involves heating the parts to tweak their molecular chemistry, making the metal more pliable and short-circuiting the 'shape memory' in its crystaline structure, so it won't 'snap back' the way you've experienced.
I'm sure there are probably helpful videos on Youtube demonstrating the procedure.
Good luck...and let is know how it turns out.