Depends what you're doing it in i spose but...
Start early and aim to finish early to leave room for amending and finalising.
Key up on research techniques before you start, ask tutors, previous diss students or master/phd students who are well used to research. See if you can obtain previous well graded disses you can get a feel for the style and such from and work out what made them so good.
Get someone to proof read, you often get so used to what you've written you can miss mistakes or things that don't make sense to anyone else.
Break it up nicely, who wants to read one continuous block of work? Break it into parts which flow nicely and take the reader through the work easily.
Make sure you keep a full log of your sources as you go along - bibliography as you go.
Be very careful not to plagarise.
Read similar works of others, see how people write that get published and aim for something that could be.
Be practical as to your subject, stick to something safe you can do well rather than take a risk on something that may be interesting but hard to source, take a good angle on or find research material.
Try to find a balance between being it too wide or too narrow. Also make you you have enough subject material for however long it has to be without having to resort to padding or just go on til you run out of words and stop.
Read as widely as you can on the subject. Remember electronic as well as paper resources.
Bear in mind who will be reading and marking it.
Hope this helps x