For the most part it takes practice. Most trickers like a very short draw, especially in competition as they are timed out for sets of tricks, this means high wattage for quick 1 to 2 second draws, if not quicker as each second counts. Most of the work is in the jaw and throat, some even say putting on an angry face helps. Other contributing factors is stillness of the air, so a well ventilated area is detrimental to the longevity of the vapor, cool area with high humidity will help immensely, also air movement if you try to push hard with your hands you create forepressure at your hands, this air can disperse vapor rapidly, so you have to get a feel for the air currents around you and you are creating, just enough to manipulate the vapor around, this will become very important when you move up to trying to do jellyfish (make a vapor O then gently try to add a second stream of vapor at the bottom of the O, blow to hard instead of just enough breath to lay the vapor onto the bottom of the O will disperse and pop the O).
Maybe trying at about 80 to 90 watts might help with more vapor production, then as you go to make your O, hold some vapor in your mouth so the VG absorbs more moisture from the saliva in your mouth.
It can take weeks of practice to get good at tricking.
