First of all, have everything prepared in advance. This is where most of the time in procrastination is eaten up. Starting writing sessions is all about flow, and you can't have flow if you have to flip between screens a lot or standing up to get something midway through. Limit multitasking to an absolute minimum, time constraints be damned. Consolidate any research to research blocks and writing to writing blocks, even if you have to alternate daily between the two.
That being said, I personally have a rhythm I try to achieve each day. Tea helps me (and many others) concentrate, but I've ingrained a little "checkup" habit over the years. If I'm stationary, my word count is coming to a crawl in the last 10-15 minutes, I have to reset, and just force myself back into the document. Writing something down takes precedence over everything, including quality. If it's bad writing, well, that's why there's a delete key.
Starting the first few paragraphs of the day is a bit trickier. I've incorporated a mini-freewriting exercise to get the creative juices going. If you don't know what that is, it's basically writing without thought. The only goal is to type, type, type, no matter what you're thinking about or how clearly your thoughts are running. It takes not even a full minute for me (although, I'm sure it varies person to person), and it really works. I imagine it much like warming up for an exercise. You're essentially stimulating your brain in a way that's prompting itself to switched gears (into writing).
Otherwise, organization is your greatest ally. Have everything you might need to consult one click (or zero clicks) away. The less you have to dig--> the less time it detracts from writing--> the more easily you can flip back to writing without stalling out for long periods of time.