In reading through all this about duct cleaning, THE ONE preventative measure people need to realize is that a poorly installed HVAC system that is not sealed and air tight to ensure that ALL indoor air passes through a GOOD air filtration system IS THE best means of ensuring your ducts remain clean. Duct cleaning does not now and will NEVER improve an HVAC systems efficiency. Proper maintenance and installation are the key just as improper installation practices allow for dust infiltration into the duct system. I am a state licensed contractor in Texas, and unless people have their hvac system sealed and/or properly installed, duct cleaning is an absolute waste of money. My duct system is 12 years old as is as clean today as the day I installed the system with ductwork. SO before ANYONE jumps on the bandwagon of duct cleaning, get with your hvac professional FIRST.
Demonstrate visible evidence of microbial growth in your duct work. Some service providers may attempt to convince you that your air ducts are contaminated by demonstrating that the microorganisms found in your home grow on a settling plate (i.e., petri dish). This is inappropriate. Some microorganisms are always present in the air, and some growth on a settling plate is normal. As noted earlier, only an expert can positively identify a substance as biological growth and lab analysis may be required for final confirmation. Other testing methods are not reliable.

Some research suggests that cleaning heating and cooling system components (e.g., cooling coils, fans and heat exchangers) may improve the efficiency of your system, resulting in a longer operating life, as well as some energy and maintenance cost savings. However, little evidence exists that cleaning only the ducts will improve the efficiency of the system.