1. Explain as clearly as possible why the change is happening.It's not enough to say 'Gmail is better' because that will always be a matter of judgement and individual preference. Be more specific and explain that Gmail will be implemented to help manage desktop and server costs, improve up-time, provide more storage and allow better support for mobile workers. By understanding these points and the bigger picture (ideally communicated from the 'business' not just the IT department) users will be much more tolerant of the impact on them personally.
For users who want to continue with Outlook it's also important to explain the risks and costs associated with the desktop approach. This isn't about choosing an email client to suit individual preference, it's about choosing an email system that works for the entire organization.