If you plan to put new decking down on top of joists, Ron offers a tip that you might want to consider.
After decking has been down for a period of time, it tends to dry out and it can slip around the screws or nails. When you walk on it, the movement can cause the two pieces of wood to rub together making a squeaking sound.
To prevent this squeaking, use strips of roofing felt, (also called tar paper) and tack them with staples to the top of each joist.
The felt creates a cushion for the decking so that it won't make any noise and it also provides additional waterproofing.