I actually decided last year that I would be Mystique for 2012 Halloween. I start preparing in April of 2012. I started with a blue full bodysuit. I then purchased the red wig, contacts, and Fila Skele-toes. I searched as many Mystique costumes and videos that I could to determine what would work best for me. I took my time and added the designers to the bodysuit. I used puffy paint to create the designs. I created a plunging neckline and add a bra into the suit. For a little texture, I unraveled and used a shower loofah sponge.
I went on to search for the perfect blue eyeshadow to match the suit. I used this to create the color. I then used Play-Doh to create the scales on my face and neck. Lastly, the Fila were spray painted and the black finger nails were added.
What worked for me was viewing as many pictures and watching the X-men to come as close as possible to the real thing. I hope this helps in creating a future Mystique look.