When I original start to build my armor I used "Sintra" ( can be obtained form plastic suppliers usually) rather than polystyrene sheets, as its not as brittle and bends well when heated. I also find poly-sheets to not be very resistant to knocks or wear and tear of trooping, as it cracks easily ( Now I have F.P armor, though a lot more expensive than making my own, it's well worth the price!)
In regards to heating the back armor, you can either do it in an oven at a low temp, or with a heat-gun, which is great for the bends as it allow you to concentrate on certain areas ( remember to wear protective gloves ). And you will need another person to help, they can help mold it to your back shape.
A good way to heat just a small area, i.e you need to bend along a line, is to block the area off that you don't want to heat with thick board; 10mm mdf is good, or something similar. Scoring the line you want lightly (this cant be expressed enough!), put the Sintra or poly-board on a flat surface (table with straight edge ) with the line hanging on the edge of the surface, place one blocking board above the line and clamp to the flat surface so the piece you want to bend wont move. Place the other blocking board on the on the other side of the line, leaving approximately 10mm of the board you want to bend showing. Using the heat gun and protective wear evenly heat the board along the line exposed by the blocking board. The Sintra or Poly-boad should start to sag a little, when it does it is now ready to bend. Because of the tables edge and the fact that the board is clamped to it, you should now has a nice straight crisp bend.
I find using the Blocking boards not only helps protect the Sintra/polystyrene form excess heating, it works as good surface to help in bending the piece as a whole, rather than in parts when using a gloved hand.
I don't really know if this will help you, as you didn't state how you messed up, but I hope it does in the future.
Good luck.