Spray the plastic and the back of the graphic with water or Windex. I've used both, Windex lets the graphic float longer, but sometimes I find it takes too long to get the graphic to stick. The best I've found is an couple of ounces of Windex mixed with 14 ounces of water.
Have a blow dryer handy to heat the graphic and make it easier to flow contours. Start in the middle and work towards the edges. Take your time and be patient before you really rub the graphic down and squeege the water out. Practice helps too. It used to take me an hour to get graphics on, now it's a 10 minute job.