Best thing to do is to keep a list of directories to submit to and blogs to comment on, and also carefully link a few of your other websites together.
Also, people post for link exchanges or offer a link in return for a guest post on here too; that will take longer unless you outsource the writing. If you have any established accounts on forums, add a link to your signature.
If you have some kind of budget and aren't opposed to the idea you could pay for sponsored posts too (sidebar/footer links will be a lot more expensive).