Use your finger, a pin or a plastic putty knife to lift the corner of the film and adhesive away from the surface. In doing so, you will prevent adhesive from remaining on the surface. During removal, you will see that the adhesive usually becomes stringy. You can save a lot of time by making the adhesive stay with the film as you remove it. Experiment to see what it takes to achieve this, then stick with the process.
Continuity is important.
To eliminate most adhesive residue, try to do all the removal in one sitting. Despite the strength of your technique, you may find that some residue remains. This is due to the wax and degreaser solvents often used to prepare a surface prior to the graphic application. More of the same. Fill a squeeze bottle or trigger-type spray bottle with the wax or degreaser of your choice.