Outline your overall story concept. Since this is a continuous story line, you won't be outlining a single story line with a beginning, middle and end. You will need to have several story lines running simultaneously, with multiple characters and viewpoints. A story like this takes overall planning. The best place to start is by creating a story overview, a character list and a synopsis of each beginning story line. Consider using a family-tree type graph system to help you visualize the story lines and the ways they may connect as the overall story progresses.
Pay attention to what you write in the present. This is important. Later on, if you overlap story lines or remove characters and then bring them back, you will need to be consistent with what readers already know. This is called continuity. Always proofread everything new you write for flaws in the continuity of the story. This means everything you write in your story needs to be in line with what has come before.
Keep the high points coming. Keeping readers with you for a continuous story can be difficult, so keeping high points in your story is crucial. Stagger those high points of tension, tragedy, romance or conflict between your various story lines so you always have something to hold reader interest.
Make notes all the time and be prepared to rewrite on the spur of the moment. The notes will be your way of jotting down possible sub plots and new story lines to further the continuous story. Being prepared to rewrite means you can let the story breathe; let the story take its own direction if it feels right. This happens often with a story -- especially one of this magnitude. A continuous story has no real set ending, so the possible directions it could go are endless, beyond what you've imagined.