How do I keep an open bottle?
Air is the enemy
If you don't finish the bottle in a single sitting it will oxidise if left for more than a day or so, so it is really best to drink it all the day it is opened.
When you first open a bottle, the oxygen first helps the wine expand for a little while, but after that it slowly ruins the flavour of the wine.
On the other hand, we are all trying to enjoy our wine in moderation!
There are three ways to slow down the oxygen to enjoy the wine another day
Approach #1 - slow down the air
Approach #2 - suck the air out
Approach #3 - displace the air with "bodyguard" gas
I have a whole blog post about these options