other than what's already been said, one great
SEO tactic that's not mentioned very often, but that I recommend is launching an image gallery on each one of your sites. If you put up a lot of images into some sort of image gallery on your site, those images get indexed in Google Image Search and I get a lot of traffic from people coming in from Google Image Search. Try that for just another way to build traffic.
Of course, there are all the other standard ways - link exchanges, link submissions, etc. In the end, if you're busy (like I am) its best to hire this stuff out to teams / firms in India where its low cost.