There is no quick fix. Endurance issues are virtually always from poor air flow. I recommend the Chicowicz Lone Tone Studies available at Balquhidder Music and Clarke Technical Studies. Like VB recommends, these are low impact studies and when done properly, are truly of great benefit. Many experts say you must rest as much as you play. Do not force anything. When you feel fatigue, stop playing. I do lip flaps, but be very careful how emphatic you are when doing them. Just a few seconds and not so hard you bruise your lips. Yes, that can happen if you are too forceful. Gentle massage also works well. No matter what you may read here, EVERYBODY gets tired at some point. Great orchestral players, soloists, jazz pros, brass band players, etc. eventually get tired, but they have trained themselves to the point it takes a while before they reach that point. Proper hard work will bring the results you want, but it isn't going to happen overnight and a good teacher is also needed. Rich T.