Ensuring a smooth ERP migration is complex, and every implementation entails a certain level of business and technical risk. There are a number of factors that affect an implementation's level of risk, including the number of sites that you are going live with, how many legacy systems are being replaced, and how many users will be affected.
In general, the variables that are most likely to reduce the business risk of your migration include:
1) Phased instead "big bang" approach to migration - cutting over your systems all at once generally increases your risk, particularly on large projects across multiple geographies/countries.
2) Sufficient Training - the better training you provide users, the less problems you will see.
3) Legacy System Planning - what are you going to do with your systems after go-live? Will you run them in parallel for a short-period until you know the new ERP system is functional? If so, have you budgeted these costs in your ROI? Failure to answer these questions before go-live will create significant problems at cutover.
4) Thorough Testing - Unit and integration testing is very important; you significantly reduce your implementation risk if you have thoroughly tested the solution with real data and real user profiles before go-live.
5) Provide Plenty of IT Support - expect more support center call volume and staff accordingly during go-live. You will also want to make sure you have clearly defined escalation procedures in place for ERP issues that your support staff isn't able to handle.
6) Develop a Contingency Plan - what will you do if your system does go down? Do you have manual processes you can revert to if needed? By expecting the worst case scenario, even though it is unlikely, you will reduce the risk of a massive business failure.
It all boils down to ERP risk mitigation, and the addressing the above issues will help minimize the level of risk exposed to your business. It is important to ensure that your project plan, budget, and staffing all consider these items.