Read as much as possible.  Newspapers, magazines, books, fiction and non-fiction in your research domain.  Good writing comes from reading good source material both in and out of your field of inquiry.  For example, I find the best writing is in The New Yorker and they cover news, design, science, medicine, fashion, culture, humor.  Reading will help all the steps of the thesis process as you create your own work.  Keep a running bibliography and see what will be useful specifically to your upcoming thesis.