I bought some tinting film off ebay for £10, when I did my Punto, it was so cheap I was able to practice, throw away any screwed up bits of film and ultimately got a very good result, I'm a bit of a perfectionist so If it wasn't 100% I wasn't going to leave it on the car.
Hopefully you can see by the pictures it came out pretty well.
Also helped in the summer keeping the car much cooler and helped the aircon be more efficient.
So to explain the pictures, you put the film on the outside of the glass, get it shaped around the glass heating it with a heat gun to shrink it to follow the contours, then when happy and when you have got rid of every last wrinkle and bubble, you cut the film along the black edges of the back window glass, this makes it much smaller than the whole window but will still tint all the clear areas of glass, you then peel it off and put it on the inside of the glass, knowing that it will fit perfectly without any problems.
Float it into place with some soapy water and squeegy the water out so it sticks.