Always expect long data
When preparing layouts for ￼MyDataMerge, you have to create placeholders. Please make sure that text frames which contain placeholders are big enough to provide enough room for even longer text replacements.
Move unneeded stuff to master pages
Instead of keeping all items on the regular pages, move everything that’s does not need to be duplicated (like decoration, background images…) to the master page. For large databases, this will speed up the process a lot.