After you have chosen your editor (or agreed to edit a product) these sample contracts can be used by the editor or the writer (client) to clarify expectations. Jurisdictions vary, so start with one closest to where the work is being done. Also look at the bottom of the list for articles relating to contract terms — explanations and considerations.
Canadian: Standard Freelance Editorial Agreement (also in French) from EAC
US: Letters of Agreement from EFA
Sample Freelance Contract from Copyediting.com
UK: Terms and Conditions from SfEP
A non-disclosure agreement (NDA) or confidentiality agreement is standard and reasonable business practice, but not all agreements are created equal. Learn what parts to negotiate in my How To column at Copyediting.com. For a guide to other contract terms, check out the book The Paper It’s Written On, by Cather and Margulis.
Here’s a Globe & Mail article about non-compete clauses and why they (usually) don’t hold up in court (in Canada).