Window Decals! 
 
 Instructions
Clean where you are applying the transfer to using an alcohol wipe or gentle dish soap, then let dry completely.
Peel the transfer off of the white/clear backing, ensuring all pieces lift off evenly. Be careful not to touch the adhesive.
Position the transfer and then slowly stick it onto the area (remember these transfers are meant to be long lasting, so apply carefully).
Once the transfer is applied, remove any bubbles or wrinkles with firm pressure using your fingers or a credit card edge, starting from the middle, and smoothing outwards.
After you have smoothed the transfer out, apply pressure using your fingers to ensure it has adhered evenly.
Finally, slowly peel the clear layer on the front. Do this diagonally (from the top left corner to the bottom right corner, for example).  If the transfer pulls up with the clear layer, push it back down and firmly press it back into place before trying to remove it again.
Note: These are NOT recomended for adhesion on vehicle paint.