This document is one of the deliverables of the EOSC Association (EOSC-A) Task Force on “Technical Interoperability of Data and Services”. Its main aim is to highlight a number of design principles and suggestions that increase interoperability and composability opportunities for data, software and services, and it is intended to complement the forthcoming Task Force deliverable entitled, "A landscape overview of the EOSC Interoperability Framework: Capabilities and Gaps". The document introduces design-stage principles, both architectural and non-architectural, that are commonly met in software design and maintenance best practices, but are in this case approached from the perspective of EOSC’s technical interoperability. These 21 principles are additionally catalogued in Annex A in tabular form including the guidelines and recommendations of this report, together with the suggested criteria for satisfying the respective principles. The document then proceeds to specify suggestions for the interoperability of EOSC services, addressing the pillars of Open Science, Cloud Computing and the User. The role of standards, as the cornerstone of system interoperability, is presented briefly, and Annex B contains an evidence-based enumeration of the relevant specifications. In the Annex, an indicative list of standard families, along with sources where common standards are drawn for EOSC services, is presented. In conclusion, a set of key points is summarised, that emphasises the need to adopt an interoperability strategy at the time of design of a given system.