A while back I was having trouble with endurance and couldn't work out why. After playing for 25+ years it was something I hadn't had a problem with before, and my teacher suggested I buy a Warburton P.E.T.E. and give it a try.
WOW! My mind was blown, with just 5 mins a day I found not only my endurance but my range improved out of site. I went from a solid D above top C, to a solid G above that, which for my playing is more than enough. My endurance went from 1.5 hours constant to 3 hours.
The P.E.T.E. is so easy and mind blowing. I strongly suggest getting one if you live in an apartment and can play your horn, or if you are struggling with range and endurance.