You are completely and utterly right about the poor reflective qualities of tin foil. Learned that from the all-knowing StinkyAttic.

If you don't feel like buying anything then I guess white paper is better than tin foil...even though it will be a pain in the ass to put up and isn't cleanable. Aside from the paper though, if your walls are already white then that's already one of the suggested methods of light reflection, just not top notch. You won't be getting top notch anyway since you're not buying anything