Finding time to manage and balance all of the activities in your life as well as maintaining a productive creative writing schedule is tough ... don't let anyone tell you otherwise.
Mostly I work on one writing project at a time, but on occasion (like now, for instance) I am juggling a few different projects.
I'm currently working my way through a first edit of my next novel, "Caretakers", and while it's exacting and demanding work, it does not provide adequate venting for all of my creative impulses and urges and juices. To keep my brain working in a creative manner, and give the voices in my head an outlet, I am additionally working on two novellas and a piece of serial fiction (not to mention my ongoing commitments to blogging, Facebook, G+, and other flavors of social media that all indie authors need to be engaged in these days).
To keep everything working as smoothly as is possible, I have a couple of rules/guidelines/tips that I try to follow, and thought that I would share them with you.
#1 - Write it Down!Often, especially, during the times when I'm editing mostly completed work, I will get ideas for new projects. It never works to hope that I will remember the idea(s), so I write them down. I keep a notebook with me at work and at home, but will also send myself emails, and sometime leave messages on the answering machine at home.
#2 - Talk it Out!
I have a couple of people with whom I share my ideas, and often find that talking about the projects I've been thinking about will help me refine the good ones and discard the bad ones; having to explain a story that I haven't yet written allows me to test the waters without a big expenditure of time and effort ... and I get to drink coffee.
#3 - Map it!
Once I've had an idea (or a series of ideas), and talked about it/them with my creative writing support personnel (guru, therapist, minion, etc.), I like to map out the story. Mapping the story allows me to see how the characters and conflict and such will work together (sometimes I know everything about a story before I start writing, sometimes I only know the starting point and a couple of markers along the way ... it varies ... a lot!). My maps range from elaborate software driven things to circles and lines on the backs of barroom napkins (yes, really) ... do whatever works for you in the moment.
#4 - Start Writing!
I'm a big fan of the "write it fast and fix it slow" school of creative writing and storytelling. Once I take an idea through the preliminary planning stages, I want to get writing. When I have time to dedicate to writing, I like to aim for at least 2,000 words per day (I often write more like 3,000 to 4,000 words per day, but I like to set a baseline of 2,000).
I'm not interested in getting the story 100% right the first time around, and generally don't go back to fix the previous day's writing (unless I got a name or place wrong, which is a simple detail-oriented fix, which I don't count). I like to get the whole story down before I go back to clean things up, preferring to maintain the flow over getting it perfect (I understand that Kurt Vonnegut spent days on each page sometimes, but that one he was done with it, it was ready for the printing press).
#5 - Maintain Your Focus
In some instances, I've found that multitasking actually translated to everything being done poorly. I wasn't able to edit my work very well, and the other writing projects I was working on weren't flowing well ... nothing felt right.
Sometimes, even if you're doing everything according to a formula or pattern that you've used with success in the past, it just doesn't produce results that you're happy with ...there's probably no metric for divining this, but you'll know (if you listen to the critics living in the back of your head ... what, don't pretend you don't have them).
If you find that your main writing project and/or the other ones you're working on are suffering as a result of you juggling multiple projects at once, then just drop the least important one(s).
In my case, finishing the first edit of my next novel is the most important of my writing projects, so everything else has to share space on the back-burner if I'm having trouble with that project.
The exception might be if a story was burning a hole in my head, trying to get out. If the story is so exciting to me, and so eager to shoot out of my fingers and into my laptop, then I might take a week off from the novel to get a first draft of the story out.
You'll be the best judge of how best to maintain your focus when juggling multiple writing projects ... keep long term productivity in mind.
My current plan is to keep editing my novel, and start work on each of the other three projects sequentially ... we'll see how that plan stands the test of time and work and life.