Incidentally, the biggest mistake I have made — and I have made it a lot myself — is accepting chapters for editing too early in the process.
In any type of publishing, the sequence of editing is supposed to go from in-depth editing to surface editing. Deep to shallow. That is, substantive or developmental editing (the kind that involves changes to ideas and their organization, research and its presentation, and so on) is performed on the rough draft. After that rough draft is revised, copyediting can take place, and then finally proofreading. In a dissertation scheme, the people who perform the substantive editing are you and your advisor. The person who performs the copyediting and proofreading is your editor (and perhaps a separate proofreader).
This means that ideally, your editor should not be receiving your draft until you are done with any and all research, writing, and revising you are planning to do.
Where the mistake comes in is that dissertation writers often — very often — want the editor to go over their draft before their supervisor or advisor has seen and commented on it. I can understand wanting to put your best foot forward immediately and not wanting your advisor to have to work with something rough and unfinished.
However, there will nearly always be revisions to make after your advisor has read this draft, and those revisions are often substantial. If you ask your editor to then edit the material again after you’ve made those revisions, she may be performing an entirely new job. This isn’t the way editing normally works, and it will be within her right and prerogative to charge you twice for a timeline like this — once for the prerevision draft and once for the postrevision draft. If the editor is working at an hourly rate, then you can imagine how fees will mount with each draft.
To keep costs for yourself to a minimum, plan your dissertation timeline far enough in advance so that it looks something like this:
(1) Write your chapters and submit each one in rough form to your advisor/supervisor.
(2) Your advisor/supervisor comments on these drafts and requests revisions.
(3a) Repeat 1 and 2 until your advisor has no more suggestions that will involve reorganizing material, rewriting material, or adding new paragraphs.
(3b) While you’re in the process of writing and revising, search for and contact an editor, confirm their availability, and find out how much time they will need for editing a full dissertation.
(4) After your substantive revisions are complete, send the entire dissertation to your editor with a sufficient amount of time to work before you need to submit the work for defense.
Editors vary widely in how much time they need, but expect as a minimum one week per chapter or one full month for a completed dissertation draft. This is the minimum I work with. Some editors work much faster than I do, but expect to pay more for fast turnarounds. Editors might also have many projects on their plates and need a longer turnaround. Be in communication before you need them.