What we did last year that worked very good was to use some thick clear vinyl from the fabric department at walmart.  It cost more than plastic but it's clear and you can't even tell it's out there.  To attach it I ran a small bead of clear silicon around the outside of the window frame and used a staple gun to staple through the silicone about every 4-6" around the window.  We are on a busy gravel road so we don't open our windows in the summer due to dust issues so my plan is to leave the vinyl on the windows until I raise a good enough crop to be able to replace them.  So far the vinyl has been on the windows just under a year with no problems and it's still as clear as when it was put on.