Drawing from knowledge gained during decades in the business:

Other than rare cases,duct cleaning is wasted money.

Some permeant filters work but they are quit different to those that homeowner cleans.

So what to do???? Glad you asked. If you can find a pleated throw a way filter (Merv 8 preferred but 6 or 10 is exceptable until you order Merv 8 filters) that fit's,install it now. The filter must fit so that air can't go around it and remain unfiltered. If you are unable to find well fitting filters,most big box and hardware stores will special order them if you ask. The rest require's you get dirty or call in a professional. Make certain intake/return isn't pulling dust laden attic air. Next,check the intake side of evaporator coil for dust and lint restricting air flow through coil. There's about 3 correct ways and more than a dozen incorrect ways of cleaning coils,make sure they are done right. Take care of that and you will notice an improvement within a couple of months and a day and night difference in about 6. You might be wondering if all your work is for nothing unless ducts are cleaned. No,and here's why. If and when dust come's off wall of duct and blow's into living space,25% settles on floor where it is picked up by vacuum cleaner,10% settles on furniture to be removed when cleaning house,1% is inhaled and the remaining 64% is pulled into return where it is captured in filter. This % never changes but gross amount progressively diminishes so that instead of 6 oz circulating first day,only 1 oz will circulate day # 180. In addition to a cleaner home,the cherry on top is ac efficiency increases and energy consumption decreases.

Everything Klem mentions is important, but for homes with what is considered 'too much' dust there could be a larger problem at play with your houses breathing or lack thereof.

When you turn on your HVAC system (cooling or heating or just fan only) the system needs a certain amount of air to be able to function properly. Filtration is important just as much as the design of your duct system.

Air in = Air out and blower motors are stupid.

I know that sounds like a tongue in cheek kind of thing.... but if you tell a blower motor to pull air through too small of a return he will still try to do it.

Too small of a return makes that motor work harder... and guess what?

If that motor can't pull air thru your filtered return it will pull air from anywhere it can thru any and every crevice and typically these crevices are dirty places. The blower motor only knows one thing -- pull / push air. It will do this under every scenario you put it under.

In some cases the return (chase) is on an exterior wall of the home and the return isn't properly sealed this can lead to outside dirty air being sucked into the home and distributed thru the house.

Filtration at some point can cause even more problems than it alleviates, if a restrictive filter is used because it allows less air thru an already too small return opening.

These problems often times put your home under what is called 'negative pressure' or a vacuum. When that happens dirty outside air infiltrates into the home making you think you have a dust problem.

This is all very subjective, because dust is everywhere. The key is determining the actual problem and fixing that otherwise all you are doing is treating the symptom which will keep occurring until resolved.

What is treating the symptom? You have too much dust and so you clean the duct system. Problems are left and merely return a short time later. (Symptom Treating)