It is best to have a licensed heating and air conditioning contractor repair your system's duct leaks.

Return duct leaks are difficult to detect because the larger return ducts operate at a lower air pressure and air is being drawn into the system. And if you only repair the supply duct leaks, even more unconditioned air may be drawn into the system.

Supply duct leaks are more easily noticed because you can feel air blowing out at the connections or see nearby insulation moving.

Duct leaks can be sealed using mastic or acrylic-adhesive foil tape. Mastic adheres well to most surfaces and provides an effective long-term seal. Mastic alone may be used to seal cracks less than 1/4" wide. Foil tape carries a 20-year guarantee if applied properly.

Any sealant should carry the Underwriters Laboratories rating (UL-181) specific for that particular type of duct. Most duct manufacturers are now listing the closure products that they allow to be used with their ducts.

If you see the contractor bringing in duct tape, hire someone else. In the past, many systems were sealed with a gray, rubber-adhesive, cloth duct tape. This tape will eventually fail due to its short-lived rubber-based glue. If you see this kind of tape in an existing home, be sure to check all areas where it is attached to the ducts. If your contractor insists on using this type of duct tape, use a different contractor.

Source: Wendell A. Porter, Hyun-Jeong Lee, and Kathleen C. Ruppert, The Duct System, UF/IFAS FCS3263, 2008.

Additional resources

http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12730 

Progress Energy  Home Energy Check-  http://progress-energy.com/