Setting internal deadlines for projects has been a huge help for getting work done. Actually writing down goals is helpful, too. Also, for me at least, I need to constantly remind myself that any writing task is going to take at least twice as long as it seems. I tend to set unrealistic goals for my writing and then get frustrated; I can't finish a whole story or chapter in a single evening of writing, but I can expect to at least complete a scene.
I do like trying to make daily commitments to my writing, usually 15 to 30 minutes. It gives me a set amount of time to write, and then a break at the end. I've played with short term and long term goals as well, but I too set unrealistic goals and get frustrated. I'm working on finding what is a realistic goal for me (not what would be a realistic goal for someone else). I did buy a calendar for this year to keep track of reading periods for literary journals so I can keep them in mind as I make my writing goals this year.