1.
Pick a topic that you enjoy but also that you won't get sick of otherwise you will loose motivation to write your dissertation.
2.
Plan out your sections thoroughly and think about what exactly will be included in each section and how many words you plan on writing to avoid some sections being left with little words to full discuss your findings
3.
Start early and keep it ticking over - most people will start early and then leave it for a while and rush the rest at the end don't do this.
4.
Make the most of your tutors and supervisors as they are their to help you and they will be marking it so follow their suggestions and instructions.