Some people just add more sugar when the bubble rate slows down, and get more bang for their buck, so to speak. I have not tried this, but tend to go ahead and replace the whole mixture. I need to do some experimenting, myself. I always use baking soda, however, like CC
The other trick is to have 2 bottles going, on alternating schedules, so you are never without bubble production. I may have gotten this from CC
, as well
Connect them with a T-valve and while you are waiting for one to get rolling the other one is under way, and vice versa.