There are simple rules, like "Don't show the same bookmark again within X months", where X becomes progressively longer the more times the bookmark has been shown. I can also flag the best stuff so it shows up more often.

I did my best to group the text of the urls, using first kmeans in php (which is a very stupid idea) and finally settling on LDA. I'd like to use hierarchical LDA to automatically discover topics within my bookmarks, but that's "Future Work" for now.