3M makes a tool for this. It looks like a click pen, but it has a small needle point on it and it's designed for exactly this purpose. I got one years ago when a place I worked was having large vehicles wrapped. There was no way they could smooth out all the bubbles on something the size of a transit bus, so they did the best they could and then went around and poked a very fine hole in the bubble with this tool and it lets the air bleed out.