Make friends; join communities, groups and specific categories as many as you can because it opens new ways to get referral. You should be very active online, helping others where they could, participate in conversations at every opportunity and recommending possible solutions for problems.
Make tagging and Bookmarking easy –
Adding content features like quick buttons to "add to del.icio.us" are one way to make the process of tagging pages easier, but we go beyond this, making sure pages include a list of relevant tags, suggested notes for a link (which come up automatically when you go to tag a site), and making sure to tag our pages first on popular social bookmarking sites.
Reward helpful and valuable users –
Often helpful or popular users will be influences and champions within your social site, devise ways to elevate them buy promoting their works on the homepage, or develop a rating system. Sometimes a quick email or note in private telling them you appreciate them can go a long way. Some folks have done that to me, and for communities I run, I do that as well. Only do if sincere. Perhaps this is not truly SMO, but it will help to keep the most valuable members of a community closer to your site.
Make a Real Profile -
Make a real profile and behave like a helper or information provider not as a Spammer. Create a complete profile that looks like a real profile and don’t forget to add your photo in your profile.