There are a number of questions you need to ask a 3PL before you consider entering into a contract with them, but here's a very important one: Does it plan for cost-out and continuous improvement initiatives?
3PLs can provide cost savings that businesses can't achieve on their own, while still maintaining a high level of service. 3PLs keep a close eye on the inter-relationship of costs and services. An effective 3PL won't view this as a one-time event or a once-a-year focus. 3PLs should be constantly striving to identify potential cost savings in a client's supply chain--not waiting for the client to say what projects to work on.
Source: Industry Week, http://industryweek.com