title_body: How to store a bottle of beer once it has been opened?
upvoted_answer: The short answer is that the beer will not last long after opening, and in most cases you are best off resealing the bottle with an airtight cap/stopper that can withstand mild pressure.
  
  Two things you want to prevent in this situation are:
  
  
  oxidation of the beer, which will change the taste of a beer.
  loss of carbonation.
  
  
  For non-carbonated drinks such as wine, a common method to combat oxidation in an opened bottle is to remove the air from the bottle using a vacuum pump.  This is a bad idea for carbonated drinks, since the lower pressure will force carbon dioxide to out of solution causing it to go flat.  One alternative would be to replace the air with an inert gas (perhaps with a product like Private Preserve).
  
  To minimise loss of carbonation, you really only need to reseal the bottle so it is airtight: carbon dioxide will stop coming out of solution once the pressure builds.  The smaller the air gap in the bottle the faster the pressure will build, so if it is a large bottle with only a small amount of beer left it might make sense to transfer it to a smaller bottle first.