Every author wants to be more productive when they write. I have already talked about two things that will help increase productivity.
First, write every day. If you are not yet writing for a living it can be hard to make time to write, but it is worth the effort.
Second, set daily word count goals. Give yourself something to aim for at every writing session. It doesn't matter if it's 250 words or 2000, set a goal and strive to reach it.
And third, be prepared.
This means several things. As mentioned in the post on writing everyday when you sit down you should have the TV/cellphone/and-preferably-the-internet turned off, be in a space that is reasonably quiet and have whatever you need (laptop, notebook, glass of water) readily available.
I am not saying that if you get thirsty you can't get up and get something to drink. But it is (usually) better if you can anticipate such a need and be ready for it. It's a lot easier to stay in the flow of writing when all you have to do is reach over and grab the glass rather than get up, go into the kitchen, find glass, get water and return to your desk.
This also means you should have some idea what you're going to write. This does not mean knowing every single word you write before you write it. But you should have a task in hand for the day. Maybe it's tackling the next chapter of your novel. Or polishing that short story you banged out last week. Or brainstorming about your next project. Having a good idea of what you want to accomplish before you start will put you one step closer to achieving that goal.
There are many daily tasks that can be done while thinking about something else. If you are writing seriously I would suggest that “something else” be the current WIP. I frequently work through plotholes, figure out hidden facets of my characters while doing mundane chores like cleaning the bathroom or washing dishes. (Personal experience says that cooking dinner is not a good time to contemplate one's WIP. Unless one likes the biscuits a little more like charcoal and less like something appetizing.)
Lastly, this does not mean that if you don't know what you're going to write about that you should skip your prearranged writing session. I sit down plenty of days and try and reach that daily goal without knowing exactly what I'm going to write until I sit down to write it. But I find I get further when I have a good idea about where I want to go (even if there are detours along the way.)