Paint walls from the top down. If you are using a roller, start by making a large "V" or "W" (3'x3'). Then fill in between the gaps, blending into your initial stokes. Always work from the unpainted into the painted area.
Bumper stickers and dealer emblems are usually attached to a car with glue. The best way to loosen glue is heat, so try heating it up with a hair dryer. Use a putty knife, with its blade wrapped in duct tape to work the label up as the glue warms up.