Just find someone who you'd like to cross promote with (Several if you wish.) I myself usually have a little format I use.
Series Name:
Author:
Quick Synopsis
And for me, what I usually do is post at the bottom of an updated post, usually saying "PROMOTIONAL ADVERTISEMENT" but I know other people will make an entirely new post, some will post on Facebook, Twitter, etc. But all in all, give a short preview of your story that you want people to see.
Unfortunately I'm a bit too busy right now to properly cross promote, but I'm sure there's somebody on here willing to help you out.