To make a living as a freelance writer you have to keep track of your daily wage. I like to determine how much money I need to make in a month, then determine how much I have to make per day to meet that goal and figure out which projects will help me meet these goals.
This month I have decided that I need to make at least $50 per day or more. To make sure I make that goal each day I assign a monetary value to each project on my to-do list.
For example, one of my projects pays $300. I can break this big project down into manageable chunks that equal $50 each. So, if I do one of those chunks, I met my goal for the day. The trick is to make sure you get all of your chunks done by the project's deadline or else you won't get paid anything.
I also apply this formula to my writing on residual income sites. For example, on Suite 101 I earn around $300 per month. My contract states that I must post four articles a month. So, I divide my $300 by 4, which is $75 per article. This only works because I am going to get paid the $300 no matter how these new articles do.
Make sure not to take jobs that will take a lot of time for very little money. This will make it harder to meet your daily wage requirements.
This isn't a perfect science, but it helps me regulate my time well and allows me to take care of my family.