i just use tape...duct tape will make it stick forever, but i just use scotch and if it starts to fall off now and again i re-tape...i use black posterboard...two 98 cent sheets will do a 55, hard to beat that price
ive read of people using things like vegetable oil to kind of "glue" it directly to the glass, but i dont like that idea personally...there has been great debate about bugs, smell, etc and i would rather not risk it. people have also used water on lightweight cardboard (like posterboard)