Sitting down to actually write is sometimes the hardest. There are a couple ways that stand out as working well.
The one that I use the most is the twenty minute sprint. Sit down, set a timer for twenty minutes, start the timer and write. Don't stop for food, water, Facebook, or research. If something needs to get added later then put it in caps. The first draft of my novel is full of LOOK UP PURPLE PLANT NAME or WHAT COLOR WERE HIS EYES ANYWAYS notes. After the timer goes off set it again, this time twenty minutes of anything that isn't writing! Some of my writer friends use the time to fill in the capitalized words and research that needs to get done. I will usually watch part of a TV show, read a book, or get up and do some housework. Then another twenty minutes of writing. Keep going until the whole novel is done!
The second option is to always write at the same time every day. For some it's in the morning with their cup of coffee, for others it's right after a trip to the gym. Some will only be able to find the time to write during a lunch break, and they make excellent use of that time. Using the same time every day makes the brain anticipate the time. It'll have story planned to write because it knows that time is coming. There also seems to be less resistance to the idea of writing. Even if there is some writers block, it's still time to write something, and the brain will fill it in.
Another option is to try ones writing ot an item, food, or drink. If you only write when having a cup of coffee, tea, while eating a banana, wearing a purple hat your brain will notice and respond by making the writing easier every time to you use the trick. There is a load of scientific research on this, and it sounds silly, but it totally works.
The last one is often talked about by big name writers who have a lot more time than i do to write every day. They sit down and they don't' get up until their goal for the day has been completed, and usually that goal is close to the 2000word mark.
Until I am a full time writer I won't be using the last trick myself. The first three I use as much as possible to get words on the page, and they definitely help!