I have a few approaches, some better than others.
The worst approach I take is to procrastinate, distract my mind from the task (and the guilt of not doing the task) until the deadline is right around the corner, then rush and do a mediocre job. I can’t really recommend this, and I try to avoid doing this often.
An approach I have been successful with is to break down the big task into sub tasks that equal 15–30 minutes, then pick a few to focus on for the next hour and do them. Baby steps, basically. This works very well when I write documents or essays – spend 15 minutes making up the outline of sections, then spend each other 15 minute block filling in a section with content.
An approach I rarely use, but use in dire circumstances, is to have a serious talk with myself, remove all distractions, and just do the work. By serious talk I mean vocalize every bit of dread I have about the task, as well as the consequences of not doing it, then showing myself that I can do the task and prove that by doing the task. It’s not fun, it’s not happy, but when push comes to shove sometimes a slap in the face is what’s needed.
The approach I use most often is the work-reward system: work for X time, have a reward for Y time. I have to be careful to not abandon work for rewarding, but the alternating of working and relaxing is often what I need to produce good quality work while staying sane.
And music always helps for me. If I have a trance station going, I can work for hours.