The biggest factor i have found, is the overall level of moisture in the car. In humid conditions, moisture accumulates over time in the cloth and materials in the car. Moisture enters the car from outside air, wet clothing, and respiration.
It sounds like you may be making a habit of minimizing the HVAC use?
I have found the best method is to leave HVAC on continually in cool or damp weather (range permitting), because that extracts the latent moisture in the car and keeps the relative humidity low. Then if you do have a maximum-range trip to take you can often get away with little or no HVAC use because the cabin is starting off dry. The dry carpet, upholstery, padding, etc can now work in your favor.
The other thing to do is to make sure the glass is scrupulously clean on the inside. Off-gassing from plastics and other materials in the car tends to create a haze layer on the window that hydrates easily and makes fogging much more pronounced.
Glass treatments can work, but make sure the glass is completely clean before application and once you start with that stuff it has to be kept up. I find keeping the interior as dry as possible with liberal use of HVAC, works well.
My old VW Beetle had almost no heat so my solution in those days was to coat the inside of the windshield with Glycerine (pure Glycerine, USP from the pharmacy). It seems crazy but once applied it "flattens" back out into a good optical surface. Glycerine is strongly hygroscopic so over time it would get diluted and drip off the glass, requiring re-application every couple of weeks or so. It was terribly messy but the best solution at the time for a starving student. I got that tip from some old pilots.