Varying the speed and pressures throughout your session is important in order to target the large and small airways rather than leaving it at one speed/pressure for the entire time.
We no longer use the speeds and pressure originally prescribed for our daughter but rather use settings recommended by Dr. Warwick (the inventor of the vest). He did extensive research on what was most effective.
If you have a HillRom~ For the most part, 10-17hz was found to be less effective than the other frequencies. His recommendations are to use 6, 8, 9, 18, 19, and 20hz for 5 minutes each. Your starting pressure is the highest you can tolerate and you reduce a little as the hz increases. In my daughter's case, she starts at 8. This will vary amongst individuals. Every 5 minutes you should pause to cough. Sometimes my daughter needs to pause more often.
If you have an InCourage vest~ Dr. Warwick recommends the automatically programmed session, which cycles through the settings that work best for the waveform used by this vest (it's different than the HillRom.) It also pauses automatically every 5min, giving opportunity to cough.
Re. getting junk out~ my daughter doesn't do well with inhalers prior to vesting anymore. Her vest sessions are much more productive if she is nebbing throughout them. Other than that she does regular huff/coughs and pauses it as needed. Sometimes it's every two minutes... other times less often. Whatever she needs to clear things out.
eta> I got much of the information I learned about Dr. Warwick from Amy's blog... link for her posts about Dr. Warwick are down on the sidebar. <a target=_blank class=ftalternatingbarlinklarge href="http://noexcusesnoexcuses.blogspot.com/2010/09/improving-clinical-trials-act-passes.html">NoExcuses blog</a>