Sitting down to actually write is sometimes the hardest part of the process. Through more trial and error than I would have liked I have found three ways to get around it, and one that I have yet to try.
The method 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 WAS 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. When the timer goes off its time for another twenty minutes of writing. Keep going until the whole novel is done!
The second option is to 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 are only able to find the time to write during their lunch break, and they often make excellent use of that time. Using the same time every day makes the brain anticipate the time. It'll have the 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 writer's block, it's still time to write something, and the brain will fill it in.
Another option is to tie one's writing to an item, food, or drink. If you only write when having a cup of coffee, tea, while eating a banana, or 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. Usually, that goal is nice and high, close to the 2000 word 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!