I also suffer from the skin reactions. Here is a summary of the most commonly suggested ways to try to overcome it:
1) Apply a medium size Compeed blister plaster to your chosen location and fire the sensor over the top. HOWEVER, this is risky because sometimes the sensor fails to penetrate through the plaster ... I used to be fine with this method but lately every single one was failing on insert so now I use method (2)..ALSO, Compeed can lose stickiness after a few days so you'll likely need to cover it in the second week (I use Opsite over the top).
2) Put the sensor applicator on top of a medium Compeed and draw around it with a pen/pencil. Pierce a circle in the middle for the needle to go through. Apply as above. HOWEVER, some people still get the reaction when they pierce a hole AND from my experience the sensors are less accurate when a hole has been pierced in the Compeed.
3) Try a barrier cream / spray (never worked for me and actually made it worse) - e.g. Cavilon or SkinTac wipes. Neither worked for me.
Good luck!