Backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in your engine. The fuel mixture leans out enough to where it fails to ignite consistently. This, in turn allows some unburned fuel to get into the exhaust. Then, when the engine does fire, these unburned gasses are ignited in the exhaust pipe, causing the backfire.

First, check the air-fuel mixture adjustment if you have a carburetor. Also if you have a carburetor, check the intake boot for a leak by spraying WD-40 around the carburetor when the engine is running to see if any decrease in RPMs occur. If it does, replace the gasket. Another area to check for leaks would be any small rubber hoses that are attached to the carburetor. However, if you engine is fuel injected, I can only tell you to take it to a mechanic as I have not worked on them.

The last thing to tell you to check would be the exhaust manifold for an exhaust gasket leak. Air sucked back into the combustion chamber may cause a lean condition at ignition and thus cause your engine to backfire.

Check the lambda sensor in the exhaust system
ecu fuel mix sync