Organization is key. Be aware of the size, complexity, and impact your migration will take on. Make a plan of attack as early as possible. In an Excel or Word document, create a checklist of things required to succeed and migrate your data properly. Don’t think of what you need to do just within Salesforce, but external tasks as well. A lot of times data is coming from an external system, so you need to understand how the data needs to be interpreted in order to be in sync with Salesforce. Understand where your data is coming from and what its purpose is. In a recent data migration, I created a checklist for each table I was loading, so when it came time to load the real deal, I had an easy step by step process to guide me through the entire exercise.