PFS is your friend.
http://en.wikipedia.org/wiki/Perfect_forward_secrecy
Even PFS isn't complete proof against surveillance. It's possible to mount a more advanced attack, sometimes called a man-in-the-middle or active attack, and decode the contents of the communications.
A Wired article in 2010 disclosed that a company called Packet Forensics was marketing to government agencies a box that would do precisely that. (There is no evidence that the NSA performs active attacks as part of routine surveillance, and even those could be detected in some circumstances.)
The Packet Forensics brochure said that government agencies would "have the ability to import a copy of any legitimate key they obtain (potentially by court order)." It predicted that agents or analysts will collect their "best evidence while users are lulled into a false sense of security afforded by Web, e-mail or VOIP encryption."