Take your ass to walmart and buy some 4mil WHITE plastic sheeting. It will reflect the light and also you won't have any issues with the sheet's porous nature (mold, soaking up moisture, mildew etc...) and it will cost you $4.88 for a 10' x 25' sheet. Over by the painting supplies usually.

But if you can afford it use the reflective mylar. It is what I use, and the difference is night and day Soooo much brighter with mylar.

http://www.wormsway.com/detail.asp?sku=SMY100