DO-254 has rapidly become the de-facto avionics hardware development guideline. Originally intended only for commercial aircraft, it has continually expanded to military aircraft, rotorcraft and even large UAV's are beginning to comply with DO-254. While some believe DO-254 is merely a corollary to the older DO-178 avionics software guideline, the truth is DO-254 has many differences. Why? Because software is not hardware, and DO-254 has particular onerous focus on Complex Electronics Hardware. DO-254 is not cheap but done properly it can be cost-effective, particularly over the typical long life of avionics systems. This paper summarizes DO-254, details common mistakes, and provides valuable insight into DO-254 Best Practices (the author is the principal author of the world's first book on DO-254 and the provider of DO-254 Training to over 4,500 engineers in 20+ countries worldwide).