Build your mask on top of an existing plastic mask. Have a look around party shops, and find one that is a similiar shape to work with. Even if the finished result looks nothing like the base mask, you've still got lines and edges to work with.
When I made the dragon for Ma Chao's helmet, the original plasticine master was built on top of a cat mask. O_o It REALLY helped me get an even curve for the face, and line the eyes and nose up.
If you're not planning to mould and cast your mask, automotive putty is my favourite sculpting material, as it is firm and dries quickly. However, it is quite heavy.
I hope this helps, or at least, inspires you!