The basic design formula is
Spring stiffness = [tex]\frac{Gd^4}{8nD^3}[/tex]
G = shear modulus of material
d = wire diameter
D = spring diameter
n = number of coils.
[tex]G = \frac{E}{2(1 + \nu)}[/tex]
where E is Young's Modulus, [itex]\nu[/itex] is Poisson's ratio.
You can change any or all of G, D, d or n.