When I finally sit down to write, I will have in my mind what it is I am looking to achieve in the scene through pre-planning in my mind. If for any reason I then find it hard to actually commit pen to paper or that I got stuck in the pre-planning phase, I will first go out for a walk. I find the gentle exercise and slowly changing environment while walking highly conducive to the thought process. This also works for me whenever there is something I need to think through or have a problem to solve.
Sometimes, though, with the writing process, it is necessary to walk away from it for a while - even months - until something clicks and you find yourself back on track.