How to maximize security in SFTP transfer?

I've been assigned to a project involves file transfers from my company to several other companies. Since I was not familiar with the concept, after some research and study, I managed to implement an application managing SFTP transfers using WinSCP .Net Assembly. Currently transfers need Username + Key File authentication and there are IP restrictions in both source and target servers.

I am now wondering if there are anything to do to enhance security?

Thanks in advance.