1) Put a link to my last.fm profile in my forum signature.
2) Ask people to review what I listen to and make recommendations.
This has been infinitely more useful than the standard machine-generated recommendations, particularly when it comes to specific sub-genres and the like. A human who sees I have Black Sabbath, Sleep, and Pentagram in my rotations but not Pantera, Korn, or Slipknot in my list is probably going to implicitly understand that I don't like nu-metal, and will recommend something within the traditional heavy metal or doom metal genres.
The machine algorithms, on the other hand, think it's all just "heavy metal", and will then send me bogus recommendations for Marilyn Manson and other crap that I'm never going to touch.