Nasal rinses. You can use saline rinses (I prefer hypertonic saline) and adding a small amount of baking soda to alkalinize the rinsing solution has even greater benefit. Or you can purchase packets at any drug store. I work in a hospital and have used nasal rinses a few times per week. Haven't gotten sick yet. [Fate don't smite me, I'm trying to help fellow travelers].
Quercetin. Taking this results in a 40% improvement in outcomes from severe infection and is 93% effective in preventing COVID infection when used as prophylaxis.
Read more about effective treatments at https://c19early.org. It's a great site to show what has worked and what doesn't work (like convalescent plasma). No medical advice, but I also think these treatments have a similar effect on all upper respiratory viral infections - not just COVID.
A couple of non-pharmaceuticals to consider:
I add a little borax, iodine and xyolitol as well.