I have to agree with Dennis on scrubbing. I normally do sensors after a shower, so I scrub the new site with soap in the shower. Afterwards, I spend a minute scrubbing the site with rubbing alchohol on a clean paper towel. I let it air dry, then scrub again. I also try to slightly stretch my skin before applying the adhesive. If you stretch too far, the tape wrinkles and folds when you relax, prying the adhesive off your skin. If you dont stretch the skin enough, when your skin is fully stretched, it will rip the adhesive loose because the tape is not as elastic as your skin.
Im a mobile forklift mechanic here in Texas, so I go from -10 degree warehouses to sweltering +100° warehouses daily, contort into wierd positions to get to bolts or wires, get all types of greasy, do strenuous tasks like hefting 200 lb wheels into my van or onto an axle, and regularly scrape my sensor on sharp edges through my shirt. Through all this, I can tell you a properly applied sensor will survive for 7 days, and an improperly attached one will fall off on day two if you’re lucky and had an easy day. For me, a good tape job looks slighlty crinkled after a few days, not too tight or loose, but has a little give. No giant ridges where is came unstuck and folded into itself. Medtronic will send you extra adhesives for free, you dont even need to tell them why. They will replace sensors that have fallen off early, even if it was due to user error/abuse. You just need to develop a sense of skin stretchiness for a given spot, and you’ll have some excellent applications.