Go to a health food store and get a small bottle of organic VG. Any time you replace a carto, lube up the o-rings and the outside of the new carto with a little VG. That should keep them from ripping when you put new ones in.
As for the o-rings ripping when you are refilling, there should already be enough juice in there to keep things lubed up. Maybe try twisting the cartos a little before you push them down?
Good luck...
The best tool Slap yo Mamma Stainless Steel Slap Yo' Mama Multi-Purpose Adapter [SYMSS] - $12.99 : Big Daddy Vapor
Budget filling tool Anodized DCT Refilling Drip Tip, Black
also any drip tips that look like these will work also 510 Ceramic Drip Tips
I just use the juice I am going to fill the tank with as lube for the orings. Soak a qtip and rub it onto the orings then use a refilling drip tip as graybuck and buzzlove mentioned. Never have had to replace orings yet. Its the sharp edge on the cartos that will tear them so using the drip tip covers that sharp edge and lets it slide through without damage.