I like to lie on my back on the couch or the bed with my head below the level of my feet. I then do some huff coughing.
Now this one's a little unorthodox, but if you have a sense of humor it works. Do you know how to make a farting sound by blowing on your hands? Put your hands together so that your wrists are touching, then put them against your face and blow as hard as you can while puffing out your cheeks. This creates the same sort of back pressure as using an Acapella or Flutter.
Crude, I know.