Well, that all depends on what kind of data was entered. Every fieldtype adds complexity - relationships, Playa, Matrix, Assets, Tag, categories - these all add new rows to additional database tables after the standard
The main thing to worry about is duplicate
entry_ids. My tactic here is to go through each relevant entry and add a sufficiently-large number to the
entry_id in every applicable table. So, say your production site has less than 100 new entries since the development database was last synced:
UPDATE exp_channel_titles SET entry_id = entry_id + 100 WHERE entry_id IN(<!-- comma-delimited list of the new entry_ids you need to migrate -->)
UPDATE exp_channel_data SET entry_id = entry_id + 100 WHERE entry_id IN(<!-- same list of entry_ids -->)
If you have relationships, other complex fieldtypes which use their own table that references the
entry_id of the entry their data is associated with, you'll need to do work on those as well.
After you've sufficiently incremented all these values, you can merge the DBs.