After opening and pouring a drink...
1. Insert deflated balloon in bottle.
2. Inflate balloon as much as possible.
3. Tie and stuff the end into the bottle.
4. Recap the bottle.
EDIT: or buy the above thingy.
Hey, are we talking science experiment or practicality?
EDIT 2: Or how about this? (Less spittle this way)
1. Insert deflated balloon in bottle.
2. Inflate balloon as much as possible.
3. Tie the end. No need to cap.