Below is a repost of an answer that I posted on Quora.... more info can be found here http://b.qr.ae/g6plY6

Start with a trial, regardless of which scenario you fit into and then convert to a full subscription.  The trial gives complete full featured access to Exchange Online (Email), SharePoint Online (Document management and Info Sharing), LiveMeeting (Web Conferencing), and Communicator Online (Instant Messaging and Presence)
Link to 30-day trial (bit.ly/fu8Bt8 - fyi you will need to create a Microsoft Live Services Account)

Next, planning is key to a successful transition.  We follow a 3 step approach to our BPOS deployments see more here http://bit.ly/gm6B8G .
Scenario 1: Start-up Company with a clean slate and no plans
for Active Directory

This is the scenario in which there is no existing messaging system in place, and in the case of BPOS the most simple.

The items below are things that you should consider and plan out before configuring the services, this is the most timely piece actual configuration is fairly painless.

Key Considerations

• Deploying the Single Sign-on client to employees desktops
• Domain setup and verification
• Account and Services Setup:  Creating users, contacts, and distribution lists. This can be accomplish through a web based admin console or through scripts.  Map this out in advance to keep the administration console as clean as possible
• Installation and Configuring of software clients (Outlook, LiveMeeting Outlook Connector, and Office Communicator)
• Mobile phones setup
• SharePoint Sites, Permissions, and Site configuration (this is a topic on it own, however a simple site can be created to get users started)
• Training to end users

Scenario 2: 5 year old, 100 Person Company with Active
Directory and Exchange

In this scenario, maintaining consistent mail flow and employee productivity during the migration is key.  The deployment of the services becomes a little more involved, however with advance planning and a methodical approach can be relatively painless.

Below are some of the key work items that should be completed to transition from Exchange to Exchange Online (SharePoint exclude due to potential complexity)

• Environment analysis
• Internet Bandwidth Testing and Needs Assessment
• Setup and Installation of Migration Tools
• Understand Mobility Requirements
• Active Directory Preparation
• DirSync Setup and Operations
• Coexistence configuration
• Client Computer Setup
• End-user Communications
• Migration Groups (recommend a test group, and subsequent group(s), the number is based on business requirements)
• Activate users
• Mailbox Migrations
• Post-Migration Services Test (Make sure Mailbox, Calendar, and Contact Info is complete)
• MX Record Switch
• Configure 3rd Party Applications

Scenario 3: 5 year old, 100 Person Company with Active Directory and Exchange.  They just recently acquired a 25 employee company that uses a POP3 email service. All employees download their emails to their client and do not keep copies on the server.

The activities are the same as scenario 2 with the addition of ...Import Email from PSTs (option for POP3 users)

For complete deployment resources check out Microsoft's Deployment Website http://bit.ly/h529bP 

I hope that you found value in this post, please feel free to ping me here or on LinkedIn() or Twitter (@dianemgallagher)