Definitely article writing but then you need to take the time to submit to quality article directories. This gets you the higher ranking pr backlink(s) you need. Also agreed, finding directories that match what you have to offer - again giving you the right sort of backlink. Posting to blogs - making intelligent comments relevant to what's being said. Posting in forums
Harping back to article writing again because it really is effective if you take the time to do it right. Keyword-written articles at the right density and imparting great information all balanced so search engines like what they "see" as well as the real people who really count. Do it right and you build your own "name branding" all at the same time. It's an art though, and you shouldn't take short cuts. Remember it's out there "forever."
There are tons of ways, but these are good ways to start.