What are the challenges when you have to migrate your data?

  1. You expect a go live without any business disruption.
    Your master data must be of very high quality to ensure business processes can run smoothly.
  2. You expect your users to adopt the new SAP ERP system on day 1.
    The lack of data quality will jeopardize your system usability.
  3. You want to avoid surprises.
    Surprises are manageable in the early stage of a project, not at the end.
  4. You do not want your Data Migration exercise to delay the Go Live date.
    Do not underestimate the importance of testing and checking business requirements along your implementation.

The traditional approach is to provide templates, fill those templates and use the LSMW tool to get the data loaded into SAP ERP.

The steps to get there are:

For SAP to SAP migrations, the LSMW tool is sufficient.
For the migration of non-SAP data, the Business Objects Data Migration tool is the way to go.

References:
Best practices for Data Migrations