I actually stumbled across this technique completely by accident. These are called "site wide links".
All you have to do is find a blog you can post comments to that features a "recent comments" widget.
Here's an example (look on the right hand side column about half way down):
Go to Google and type "filmmaker magazine". Click on the #1 result. I would post the URL here but am not allowed to post links yet.
What happens on these sites is, any time you post a comment, your link will appear and hopefully be indexed on every single page on the site. If the site has hundreds or even thousands of pages, that's how many backlinks you can get.
In many cases (it's happened to me over and over again) some of the pages will have a high Page Rank, so you'll get lots and lots of link juice (so long as the comments are "do follow").
I usually find these sites by accident and you will too. So keep a sharp lookout.
Now, the only problem is, your links won't last forever. If the blog is popular your comment will soon be replaced by other comments.
My suggestion is to contact the webmaster and ask if you can buy your way into the "recent comments" widget permanently. Not all of them will be open to this, but some will. Or, as an alternative you can ask to buy a link in the footer of the blog, which will have basically the same effect.