Sage advise, wouldn't you say? But, what constitutes bum glue? For me its four things:
1. Have a regular writing time. Are you an early riser? Or, perhaps you're a night owl. Regardless, set a regular time for writing and stick to it. Even if you find yourself dreading the time, once you begin you'll more than likely become immersed in the work.
2. Set a daily quota. This can be in time spent at the keyboard or word count. If you're a prolific writer, churning out hundreds of words in a short time, then maybe a word count quota is your cup of tea. On the other hand, if you're a plodder, fretting over each word on the page, then you might work better with a time limit.
3. Work from an outline. Unless you're a literary genius on the level of Stephen King--outline! If you think of your writing project as a building to be constructed, the outline is the set of plans. By outlining you're less likely to face a blank screen wondering what to do. You'll have a specific task set before you at each session.
4. Don't be afraid to kill your darlings. If you're a fan of Game Of Thrones you know exactly what I'm talking about. George R. R. Martin may be the king of killing his darlings. Briefly, what I mean is not falling in love with a character or idea so deeply that you keep you literary paramour to the detriment of the story. Remember, the story is the important thing.
Writing is hard work. Don't let anyone tell you otherwise. But, by applying a judicious amount of bum glue you can finish that project.