Turn off the power to any units that are connected to the ducts. If you are unsure how to do this, simply turn off the power to these units at your electrical panel.
Remove the tape or clamps that are holding the ducts in place. Carry each unit outside very carefully and shake it clean to remove dirt and dust.
Clean dirt and debris from inside of each dust using a very soft duster. Be careful not to puncture the sides of the duct, as they are often made from fairly delicate materials.
Vacuum the area where the ducts connect to cooling units or supply grills. Use a small attachment on your vacuum to remove all dirt and dust from these areas. You may also wish to wipe the supply grills with a damp rag or sponge to remove surface dirt.
Reattach the ducts to the cooling unit and fasten them in place with foil tape or metal clamps. Make sure the ducts are secure, then repeat this process to secure each duct to supply grills or other outlets as necessary.
Dust the outside of each duct using a soft feather duster. If your ducts have a PVC coating, you may also wipe them down with a rag or sponge and any regular household cleanser.
Consider replacing flexible ducts instead of cleaning them. These units are very inexpensive, and are difficult to remove and replace without damage. Check prices at your local home improvement store if you need help making this decision.