1. Have a Strategy
A migration is important, so treat it as a separate project, not simply something on your to-do list!
And it's not only an important job, but a complex one. Don’t underestimate the scale and work involved, from data estimations all the way to change management communications post-migration and everything in between. Before you do anything, sit down and plan accordingly. Figure out exactly what you want to do and how to do it.
How will you migrate the data? In smaller, manageable chunks or in one go to get it over and done with?
When will you do it? Is there a licensing deadline you need to meet? Do the majority of data moves need to happen at the weekend when there is less disruption to the business?
Who will do it? Is everyone working on this or will you create a specialized team to deal with it? Do you need 3rd party expertise or consultancy to help you execute it?
Whatever you decide, everyone must be on board and up-to-date with the plan.
Estimate the time and budget for the migration and then add extra in case of any setbacks. Anticipate downtime, how that will affect your business, and what you can do to minimize it.
Planning a migration is half the battle, so avoid rushing into anything.