I'd suggest deciding on a project, figuring out what all the parameters are, and set milestones. Try to decide how long, in your experience, it would take to get to each milestone. This also helps you find missing places, other flaws in your design, and it acts as a freebie brainstorming session since you're looking at things a different way.
Write out your plan. Cheat a little make make Milestone 1 fairly easy to reach. Each milestone should be a few dozen hours' work, and you should have a dozen or two milestones.
This way you see a progression of work done, you see the work remaining, and that motivates you. You know how when playing a game with experience points, you will push a little at the end of a level to get it done? Same with games that have distinct zones, you try to finish a whole zone or a level of the game before you finish your sitting.
If that doesn't work, try motivating yourself by giving a reward for when you reach certain milestones. If you're accustomed to eating out at work, stop doing it and just pack lunches. If you reach a milestone that day, you treat yourself with lunch out. That sort of thing.
Keep a notebook or a smartphone with you to record your ideas. I constantly have ideas that fly away never to be seen again just because I didn't write them down right away. Having a well of ideas to draw from makes you excited about writing. You will often develop a little kernel of coolness into an entire finished thing (a magic item, a spell, a room description, a monster, etc).
And take breaks here and there. Don't let yourself get burned out. On the other hand, if you feel like you might want to write, especially after a dry spell, YOU MUST WRITE because it will break you out of your funk.
Hope this helps!