SenorMeowMeow

Yeah I'm running a keg atm and I'm having this exact issue. 
 
I'm going to try adding more slip to my solution and just spray less on the window when I want it more tacky and see how that goes for now.

I used the reusable ice bricks, never ice, because at the ice melts, it'll change your slip. 

If you try to precut to fit before shrinking, there is NO room for error.  Most errors come at the end of the shrink not the beginning(i.e. = where the edge of your pattern is has the most room for mistakes).  Most cars leave enough room around the edges to be able to shift the pattern if there is a mistake at the end of the shrink.  Not possible if you precut before you shrink.  

You still need to cut it, either before or after shrinking. To get an accurate cut it's better to have the film laying flat on the glass in final position.
 
If you're running your film through a plotter first I wouldn't peel away the excess until after the shrink otherwise the edge might furl and burn. Then you may be left with a short pattern.