Writing isn't always just sitting down at the keyboard and pounding out words. There's research, outlining (if that's your method), revising and editing, and submitting your work to markets. Plus there's also reading (need to keep up with what's happening in your genre, after all), blogging and/or Twitter (preparing your posts and reading others'), and, if you belong to a crit circle or workshop, critiquing others' work. How do you find time to do it all, especially if (like me) you work full-time and have a family?
If you run out of time before you run out of things to do, then it helps to set priorities. While I'd like to offer some guidance about how to set priorities, flexibility is key. Some days your crit partner may need immediate feedback, while other days your blog may be more of a priority. Sometimes it's more important to revise a story instead of drafting a new one.
Lists can be a good way to manage projects. Writing things down can help you decide how urgent a particular writing task is--or at least help you remember everything you need to do! For me, marking items off a list gives me a sense of accomplishment.
Another way of managing priorities is to match your activity to your energy level. For example, if you know you have the most energy in the morning but less at night, you may want to save your proofreading for the morning and your social networking for the evening.
Finally, if you find yourself hit with a lot of things to do at once, you may want to find ways to add more writing time to your day. This may be something simple, such as choosing to skip TV for a few nights, or something bigger, such as asking your family to give you some space for a few hours or even using vacation time from work. When you're used to working for an hour or less each day, freeing up a half or whole day can be freeing. However, it's also tempting to procrastinate when you have more time than normal, so you have to take care to avoid that trap.
How do you set your writing priorities?