Flat microbrew the day after.....

Well sometimes I stop by the local microbrew and bring a gallon of it home. They pour it into a milk jug and put one of those push on type caps on it. It stays good all evening long. Now, what if I wanted to save some for the next day? I've done that before and the beer went flat. Now knowing what I know from reading here and bottling my own beer, what could I do to get it from going flat? Add some corn sugar and get a tighter seal?

I would suggest getting 2 half-gallon growlers and bringing them with you when you go to get beer. They shouldn't have a problem with it since you're saving them from using a jug, and you get to drink half a gallon one night and the other half the next night. The Flatbranch brewery in Columbia, MO sold their beer in growlers when I lived there, and the beer definitely holds for a couple of days.

You can buy them here... http://morebeer.com/product.html?product_id=17106

Transfer it to a 2L bottle and buy a carbonator cap. Attach that to your CO2.

http://www.weekendbrewer.com/carbonatorcap.htm

Thanks for the info.