The best ways to improve WiFi reception is through antenna positioning and using reflectors to guild the signal.
I have a PCI WiFI card that's positioned in a terrible location. It's in the bottom slot, and is surrounded by coiled up power cords, computer cables, and other crap. So, my signal is weak. I get "2 bars".
On a lark, I took a big potato chip bag (Lay's) and cut it into a big rectangle. Then washed the oil off, and taped it to the wall behind the WiFi router's antenna. I got an immediate one-bar improvement in signal. It wavers between two and three bars now.
The bag has a thin layer of foil, like most chip bags nowadays. That foil works as a reflector, bouncing the signal in my general direction, and also reducing interference from the other side.
There's no curve in this version of the reflector. A curve would tend to focus the signal, and thus narrow it's coverage. I didn't really want that because the signal's used throughout the house.
Eventually, I'll glue this foil onto cardboard, and add a very slight curve to it to direct the signal a little bit more toward me. Mounting it on cardboard would remove some of the crinkles, which are probably distorting the signal.
Also, using old soda cans, I'll set up some parabolic reflectors for my wifi card's antenna, and also figure out how to deal with the wire and noise problems. The real solution is probably to get an antenna that can be positioned higher up.