EMI interferance can be suppressed by a suprisingly small amount of material, hence the tinfoil example that you stated. While this is crude it is effective.
There are a number of Comercial EMI suppression solutions that can vary greatly in cost and application, some are as easy to apply as paint!
Here are some link to get you started:
http://www.mgchemicals.com/products/841.html
http://www.cpfindusprod.com/AGHT/aght.html
Metalized window tint may also be another solution, though I haven't tried it myself, I know of a freind who had trouble getting his garage door opener to work through his metalized front window sticker..... Which is odd because I have also heard that a case with the side panel off it would cause a garage door to malfunction, a weird quinkydink don't ya think?