Olfa SS AB-50S Snap off cutting blades are a tinter's best friend.  Window film installers get a clean fresh blade each time they snap off a new one.  This comes withb 50 blades with 13 snap offs per blade to be used in your Olfa knife, red dot knife or equivalent.

Considered one of the best brands in the industry this is a 50 pack of Olfa Stainless Steel Blades to replace your cutting knife blades; each blade has 13 snap-off blades. 

Typically, whenever you start a new window you should be using a fresh snap-off.