First, I went to the "Wallet Details" page of https://www.bitaddress.org/ and entered my private key. I printed the page on my laser printer.
Then, I took a piece of black paper, cut it in half, and used half of it to cover the bottom half of the page where the private key is displayed.
I then laminated them together at my local Fexex Office location for $1.99 USD per page (they do not accept bitcoin). This sample is for the bitcoin address 1Fj3iqvDoFc74QyKpReW5m2eKdP2iauiNd, and it looks like this:
The resulting page is waterproof and should keep your data safe for a long time, though it obviously is not fireproof. When it comes time to spend the bitcoins, you can cut off the edge of the lamination on the bottom half of the page and it opens up to reveal the private key:
I did this with only moderate operational security. I didn't make an offline copy of bitaddress.org and check its hash. My computer is connected to the internet (behind a firewall), so if a hacker has control of my computer they can get access to my private key. I have not worried about clearing my printer's memory, so perhaps someone who steals my printer will be able to access my private keys. Anyone who gets their hands on my paper wallet can spend my bitcoins; for more security use Shamir's Secret Sharing so that, say, 2 of 3 printed pages are required.