Schedule breaks during long periods of writing. Distract yourself for a few minutes every three or four hours. During the break walk the dog, read a blog or do anything other than writing.
Reward yourself. Set writing goals and celebrate when you achieve them. Rewards should be tangible, such as a coffee date for every chapter or a handful of chocolate chips or your favorite treat for every ten pages.
Write even when you don't want to. If you can't bring yourself to work on your novel, write a grocery list, a letter to your grandmother or free write until you get past the writer's block.
Store your writing in plain sight. Looking at your manuscript every day can encourage you to pick it up again.
Clean your desk. An organized workspace can reduce stress.
Take a writing vacation. Schedule a few days or even a week to avoid writing, and only attempt to write once that time has ended.
Allow yourself to write poorly rather than stressing over perfection. You can edit your writing later.