I'll just add to the foil part. Buy a 99 cent can of high gloss white spray paint from Lowes/Home Depot. It will be 100x better than tin foil. Even if you somehow iron the tin foil flat, it won't be as effective. Mirror (as mentioned) is probably a step better than white paint.
I vaguely remember reading somewhere that the screw-in PC
's just don't perform like a standard tube; probably because of the surface area of the bulb, if I am remembering correctly. I'll try and dig up where I found the info, but it may be a shot in the dark.