Make sure your seams run vertically like the pages of a book not the pages of a calendar. Any other direction will cause a visible seam. Even then it's not perfect. It also has a bit to do with how you layout your lightmap UVs.
Read more about the issue from this thread
After using lightmass to bake lighting for a bit now I'd say sometimes it helps to not split mirrored lightmap UVs, but to merge them instead.