I try to keep the dark cloth over the slot as much as possible, no matter where I am. I also try to not apply any fore or aft pressure to the dark slide when extracting (slowly).
On particularly old or worn holders, I pull out the slide until it just clears the holder aperture and either scribe or with a paint pen, draw a line across the slide flush with the top of the film holder. Then I draw a "V" with it's base touching the line; oriented toward the top of the slide.
When I start to retract the dark slide, the "V" gives me a visual means of judging when to stop before extracting it fully. After exposing that side, I don't reverse the dark slide, but push it back in and place a post-it-note on the surface to show it has been exposed.
I have been toying with the idea of 3D printing small colorful clips I can place on the handle of the dark slide to denote exposed film, as sticky notes can fall off.
Tried pulling apart a wooden film holder to repair the dark slide light trap, but found they are very fragile and it still leaked after the repair attempt, so I won't be wasting another half-day doing that again.
In any regard, good luck on defeating the light leaks.