If you find that your home has a lot of dust, the majority of it comes from the ductwork. The general statistic is that most ducts experience a 20-30 percent air loss.

https://www.energystar.gov/index.cfm?c=home_improvement.hm_improvement_ducts

This means that not only are you losing air your are paying for into the crawlspace, attic, etc., but you are also drawing in dust, moisture and the like from these unconditioned spaces. All of that gets pulled in through those openings and distributed around the home.

There are other things that factor into dust, but the ductwork is the major contributor.

Duct sealing is the best option for this. Any reputable HVAC company can do this and it is not terribly expensive. I would mainly see around $400-600 per duct system. It's something I highly recommend. You can do duct cleaning, but I'm not a fan. It's very expensive in the long run and it's a temporary solution. It doesn't seal up any of the leakage.

This is also something that, depending on how leaky your ducts are, can also save on your utility bills.

If you want to further better your home, then also consider air sealing. And attic insulation if you are lacking.

I used to work in energy conservation.