One of the things that causes littering is a lack of trash cans or a lack of non-filled trash cans. Not all littering (or throwing recycling in the trash) is done out of pure malice. If you make it easy for people to throw trash out, they generally will. If you make it hard to throw trash out, many will litter.

Stronger fines and other enforcement would help, but the first thing is to make sure that you actually have accurate places to throw stuff out (or have the culture where people bring stuff with them and throw out at home, like some non-American countries). This article mentions how the streets got really bad during the pandemic when the city stopped doing certain things.

SF, NYC, and other cities with trash problems could significantly solve this just by having more trash and recycling cans and by changing them more often. You'll notice how much cleaner DC and its surrounding urban areas are than SF and NYC and that's because they have a bigger commitment to making it easy to not litter and they do more cleaning.