I found this on another forum and the person who submitted it got pooh-poohed a little, but I decided to try it anyway. It's working for me.
Use lightly packed dark brown sugar instead of white sugar in the recipe that most of us seem to be using. I've no doubt I will need to redo much more often, but after a week I'm still getting a bubble every 2-3 seconds. This is more like what I was getting during the summer when the house was warmer. Now that it's cooled off, I was only getting a bubble every 7-9 seconds with the white sugar. My plants have really perked back up.