Strengthen wifi signal -- very cheaply

I got this from one of Lifehacker's lists below. It certainly looks like something I'm going to try just to see how strong a signal I can get upstairs from the router (currently goes from 100% strength beside the router to as low as 20% upstairs).It will also be interesting to see what other wifi signals I can pick up around here. At present, I can only ocassionally get just one more and that's very weak. Of course, I don't want, or need, to use them but just to see what's possible.

Here's another link with some useful tips.
And more here.
This one gives some detail on the tinfoil antenna.