If it's a sculpt reduce the vertex count to a manageable file size before exporting.Optimize the high-resolution model to speed up bakes, to avoid running out of RAM while baking, and to keep 3d file sizes manageable.DigitalSculpting and/or Subdivision Surface Modeling are the usual technique for building high-poly models for baking normal maps.A Practical Guide On Normal Mapping For Games has a detailed survey of normal mapping workflows.