When I blow a bubble, I close the tube (usually with tweezers) and I am left with a rather thin front. I hold the front into the flame, until I have so much glass gathered, that it is as thick as the wall thickness of the tube. If I have a little too much I give it a little blow to dome it. The idea is, that the tube and the closed front are the same wall thickness. Once you have this, adjust to a big bushy flame and heat the whole front to about an inch back evenly - take out of the flame - wait a sec and then gently blow. You can always reheat and blow more, but once it is too thin you lost it.
To get an even bubble you have to start off with an even wall thickness!
As to the air trap, I heat the tube until it is soft, then I strech it to make it as thin as possible, blow the hole and burn off the tube - exactly like Talonst said - I do it the same way. When melting in the lip, focus the flame on the lip and more on the outside of the lip and hold your ellbow down to let gravity work for you
