How to provide SaaS integration with enterprise systems (from the SaaS provider perspective)?

Question

I'm doing some general research for a potential SaaS project. The solution we are considering creating will need data integration capabilities with various enterprise systems.

I understand that SaaS adds complexity to enterprise integration since it lives in the cloud and outside of the firewall.

I've read a few articles that describe approaches for enterprises to integrate data with SaaS solutions. Integration approaches range from the primitive FTP transfers, custom point to point integration, to a vast and growing range of commercial solutions (appliance, cloud-based, and EAI).

These articles are focused on the customer perspective. In other words they are intended to help enterprises better understand their options for integrating with SaaS providers.

Can anyone provide some insight and advice from the SaaS provider perspective when it comes to making their solution as easily integratable as possible?

I assume the SaaS provider needs to create and publish web services API's and RESTful interfaces. Any other advice or resources would be most appreciated.

PS: I realize saying "need to integrate with various enterprise systems" is incredibly vague.