To fill, a syringe is hard to beat, once you get past any possible squeamishness about it (you can get them with blunt needles). In a pinch, you can carefully slide a paper clip down the inside of the shell. As you drip juice in, that will help it run down further. I try to let them sit a while before firing it up. If it hasn't soaked in to the coils, you can burn it too easily.
I store filled ones (that I plan to use soon) with condoms on both ends, and horizontally, thinking it would keep the top coil filler wetter. I have been told it does not help. It doesn't hurt, as far as I can tell!
For cleaning, I cut up a condom, zip-tied it to a syringe, can flush out one in 30 seconds. I use iso alcohol, it seems to cut the gunk, dries faster than water, and it won't mold. It's
also fast to fill cartos, but I mainly use it for cleaning.
A clear one can get looking pretty gunky, I can get it back to bright white with just a few swooshes with a 20 ml syringe of iso.
A fresh DC Smoktech on the device, and a spare filled one is my "go to work" rig. I vape it moderately till noon, then I just swap to the other one. I don't wait for it to lose flavor. After work @ home, I vape with something else, then I top them both up each night.