We probably can’t ensure, but use a Project Breakdown Structure (commonly but misleadingly called a Work Breakdown Structure) – PBS/WBS – rigorously developed in accordance with the rules, as the management framework for:

• scheduling
• costing
• definition of deliverable and constituent products and services, some of which will be the subject of contracts with suppliers, through mutually consistent requirements
• assignment of responsibility
• risk analysis 
• measurement and reporting.

Ensure that contracted elements are configuration items (i.e. at least with baselined and change-controlled requirements), and except for smaller, simpler elements, are PBS/WBS elements. 

Contracting against objectively well-defined and mutually consistent requirements sets will help a lot. Mutual consistency of requirements is fostered by ensuring that requirements defining any product or service for supply are the product of sound design of the bigger system of which that product or service is a part of the solution. Do not rely on just “eliciting” requirements. Implementing requirements traceability in design between requirements on the parent system and those on solution elements, including contracted elements, also helps a lot, both initially and over time.