Tip One: If you are on a writing deadline, quickly plot out your work, then take a break, come back to the page and start writing. Never be indecisive if you've got a tight deadline. It's better to write, write and write some more - then edit with an hour or so to go, than to leave your work till the last minute and have no time left to polish it.
Tip Two: If you find yourself staring out of the window, rather than writing, then engage your imagination and begin to doodle on some blank paper. Then, let word association take place. Like that classic TV show Catchphrase, just say/write what you see. You may be doodling houses, boats or people - or making up words. Letting your imagination take over gives your conscious mind a break from channelled, focused, creative work. You'll soon find that you are back on track.
Tip Three: If your deadline is three days away, write a lot on the first day, then take a half day off on the second day to let more ideas come to you, then write for a full day on the third - leaving an hour or two to copy-edit. This works especially well if you are writing for a short story competition. Never write longer than a few hours at a time though and remember to take short breaks throughout the day, so you don't get eye-strain when working on a computer or other device.
Above all, don't let your deadline be your Boss - if you plan well, then you'll be finished way ahead of your hand-in time.