You can not reuse a cork, even if you just skewer it 1/2 way with the cork screw so its still intact on the bottom half (no hole). What happens is that the cork once it gets compressed and inserted into the bottle basically takes the shape of the bottle neck. Once you remove it it will never go back in and seal completely like it did that first time no matter what.
Yep, learned that the old fashioned hard way doing some "sperimenting" on a few bottles. When opened they were all pretty much dead, and had lost their body, bouquet , finish.