Number 1 spot: Mylar (super expensive futuristic reflective gear)
Number2: White paint

You shouldn't use foil because the crinkles that will form will concentrate the light, and produce 'laser beam -like' rays of light that will burn holes in your plants.

Here's a kick-ass write up by Zandor of 'why use mylar'
http://boards.cannabis.com/showthread.php?t=37730

Mylar dude.