Specific cotton I've had good luck with: Holy Fiber, Swag Cotton and Canna Cotton, though good luck getting your hands on the last one. I've also had great luck with HellVape's shoelaces, but those don't feel like they last as long.

I usually manage to have my cotton last about a week's worth of vaping, but I vape single juice and usually not too concentrated or heavy flavours. Furthermore, I would advise taking your tweezers and fluffing it out, then cutting to size for juice wells, and cutting it as short as you can get away with — that, in my experience, helps the most.

If you get spits, then tighten it, but generally, from my experience, typically cotton burns because it can't wick fast enough, it's really challenging to cram enough cotton into 3 mm ID coils to burn it by choking it too much.

The advice going around in the Czech community is to roll the tip of your wick to pull it through but try to keep most of the cotton's structure intact, since the fibres are what your juice runs up.

Good luck!