Make a plan for you fic, outline the main events and your characters' beliefs and motivations. The times I didn't plan were the times I ended up panicking and leaving the fic aside for days because I didn't know what to do with it next. Also, try writing something every day. It doesn't have to be thousands of words, but just a few hundreds or less: sometimes I quit after 50 words, sometimes I go on and on over 700. Choose a time and place and make it a routine.