When I truly *need* to write, 1 thing that is important to me is environment. It may still be hard to get started, but once you take the first few minutes to make yourself start, a good environment can help you stay in "the writing zone". Of course, that will be an individual thing that you may have to hunt for.
For me, it means no interruptions (from people, phones, etc), good music that is slightly energetic but not overly so and no words/singing. I also do best in blocks of 2-4 hours so I have time to get into the zone and stay there long enough to really produce something but not so long as I become uncomfortable -- your brain needs breaks too occasionally. Also, don't overlook your sense of smell; generally that means avoiding distracting smells, but perhaps there are also smells that you find pleasant and puts you in a good (or relaxed or whatever) mood. There are times a good beverage is a positive, but not too much as that causes bathroom breaks at inopportune times. ;)
There's a lot of trial and error, but that's life sometimes. :) Rewards don't work for me, but I could see how they might for some.
HTH and that you find what works for you,