When your Dex says "Change sensor soon" (or something like that)....DON'T DO IT! Instead, activate the "Stop sensor" option, and once that is completed....activate "Start sensor". The old sensor will then begin just like it is a new sensor. Most of my sensors have lasted 12-15 days. I stop and change sensors if my numbers have become way out of line and they keep on giving me lousy results. That has happened for me at about 12 days, and other times at 14 or 15 days.
Yup, what Richard said.
Or just wait until the Dex says change sensor now. The "start new sensor" is available again, just click and go. I will also get a good 2 weeks with pretty stable readings, by the 3rd week (second restart) I tend to not catch highs and the numbers change slower (i.e. finger stick may be 70 but sensor still reads 120 and is slowly trending down.) You just need to find your own pattern of "sensor decline" as I tend to baby mine along for as long as I can!