Dexcom during my initial training was that the skin you're inserting the sensor wire into should be completely clean of everything - oil, lotion, and even antiseptic wipe residue. If not, the sensor won't read correctly. (Sometimes, it doesn't read correctly anyway, but that's another story.)
This is hard to do, though, since something like those sticky, yet helpful, antiseptic wipes (I use the IV Prep ones) are exactly what I need to use to help the sensor's adhesive stick to me for more than a few measly hours.
What I end up doing (and I'll bet I'm not the only one) is to first wipe the whole area with an alcohol swab, let that dry, and then make a swipe in the shape of an oval (roughly the size of the sensor) with the IV Prep wipe. This leaves a small area in the middle, which is clean from the alcohol, and untouched by the antiseptic.
The biggest challenge here, aside from the part where I need to be okay with this huge needle jabbing into me, is that after the antiseptic dries on my skin, it's really hard to tell where that middle spot is. I often need to have A come look at the area for me, from a different angle, to tell me where the non-shiny part resides. When I did this on Monday night, it came out this way: "Um, I can't see it anymore. Could you come here, and tell me where the donut hole is?"
I have no idea why I called it that, but think I'll be working that term into my regular vocabulary.