Set a goal to do one small part of it in 5-10 minutes, then the rest usually follows. If it's a real struggle to get that small part done, examine why I 'need' to get it done in the first place. Deadlines are a huge motivator; I find external ones if the task doesn't come with its own.
Also, making tea or a latte before getting down to work can help me focus my mind, over time I've come to associate both of those activities with 'focus time'.
Edit to add: Isolating the task from distractions kind of goes without saying, but I find it helps to frame the distractions as much as the task. Why am I so compelled to go play game X, or read book Y, instead of the task at hand? Going a level deeper and figuring out what your brain's avoiding/being lured by has been really useful for me (especially while dealing with focus issues after a concussion).
Also, making tea or a latte before getting down to work can help me focus my mind, over time I've come to associate both of those activities with 'focus time'.
Edit to add: Isolating the task from distractions kind of goes without saying, but I find it helps to frame the distractions as much as the task. Why am I so compelled to go play game X, or read book Y, instead of the task at hand? Going a level deeper and figuring out what your brain's avoiding/being lured by has been really useful for me (especially while dealing with focus issues after a concussion).