In the first place, there is absolutely no benefit in saving this little disc of foil anyway. What are you going to do, keep it to remind you what a great bottle of wine it was? Only some foils have the winery’s name on it and I have yet to see any that reveal the variety and vintage of the wine. If your memory is good enough to remember all that, you don’t need the disc. Want a memento? Use that cell phone camera to grab a digital image. Need something more material? Bring the bottle home and steam the label off. You can mount it in your pink wine bible.
The problem I have with the traditional method is that you will be dulling a perfectly good blade for no reason. If, like me, you have a really expensive, incredibly sharp pocket knife on you at all times (given to me by my attorney no less) you would never dream of defiling such a fantastic tool for such a mundane and pointless purpose.
My friends, there is a better way. And like all great solutions, it is elegant and simple. And it will work with any foil- even those tenacious plastics. And it will satisfy those wonks who want their foil disc keepsake.
Get a good grip on the bottle with your off hand (left in my case). With your other hand, flip your knife open. With your thumb on the back of the blade, position the knife just below the bottom edge of the foil at a shallow angle to the bottle.