My instructor gave me an invaluable tip on getting the low's locked in. You can do it every time you warm up, takes less than a minute. Start at low open C, and blow-breath in-blow-breath in-blow-breath in-blow at about 60bpm (one blow-breath in per beat) at least 4 times or until you get good tone. Then move down chromatically to Gb repeating above. It is simple, quick and works...the breathing in between notes helps you 'reset' your low note muscle memory.