I've stated this before but for the sake of repetition it probably ought to continued to be discussed. Pardon my dogma. It's a little like breath support. You have to HAMMER the concept into yourself and your students. If you're lucky you at least reach yourself. We can't control what others think or do. So: There are three techniques of doubling on low brass. 1. Maintaining solid chops on your trumpet. 2. Developing chop technique on the low brass instrument. Usually the valve, slide Trombone or Euphonium. Sometimes it includes Tuba and bass Trombone but this is rarer. Regardless the ideas apply similarly even with the double octave lower brass. 3. Switching BACK TO THE TRUMPET. Switching from trumpet to the trombone isn't included as this is the easier leg. Requires little maintenance once some kind of technique is developed on the lower brass. While some trumpet players never totally develop the full characteristic tone required for a professional sound on the trombone this is more a matter of patience and skill development. As well as ear training. The facts are that almost no amount of trumpet playing will much alter your ability to blow the trombone. However the reverse is not so. The real effort to consider must always be number 3 above. When learning or maintaining the low brass be very patient and methodical with yourself. Do not play more than ten minutes the first day you blow one and avoid playing your trumpet at all the remainder of that day. When ramping up your hours on the trombone (or similar mouthpiece instrument) be careful of the cumulative weekly load you place on your chops. I've found that in order to play well enough on a gig with BOTH the trumpet and trombone I simply must have two low brass playing commitments per week a month or so prior to doubling "on the fly". However the ability to pursue two weekly commitments (either community band or a professional R & B is what i do) requires up to several months preparation in advance as it is. So I might take a whole four months to slowly build up to doubling on the fly. This takes patience and perseverance. I gained that ability the hard way: After many episodes of temporarily blowing out my trumpet chops. No fun to be in a rehearsal (and even worse a gig) and come up flat on the trumpet when everyone depends upon you. So through these experiences a trumpet player learns that the easy way to do doubling or anything for that matter is to be patient and plot a course. A strategic plan that is. Right now I've forced myself into a community band playing Euphonium on Tuesdays. The next day like today i intend to not play any brass instruments at all. Then Thurs. eve will play both the trumpet and slide bone. Then go easy on Fri. while practicing normally through the following Monday. It's a PLAN. By sticking to this it becomes impossible for my chops to weaken. I only add solidity to the whole picture. The big problem is that the action of the larger low brass instrument is like putting meat tenderizer on your chops. The trombone mouthpiece itself is not usually harmful but on an unpracticed player it opens up the whole vibrating area of the embouchure to swelling. While excessively swollen the embouchure is much more subject to injury upon return to the much smaller trumpet mouthpiece. This creates MORE SWELLING and MORE INJURY. Thus we see the the warnings from teachers and students "not to double on trombone". Or "I never had the chops for doubling". It isn't that we can't or shouldn't double. Just that the potential doubler doesn't usually understand the over-all effect these changes place on his short term and long term brass playing maintenance. However once one has become patient and plotted a secure forecast of doubling + rest as described above the swelling from playing the trombone completely subsides. Then the chops actually become stronger over-all. So the idea of "doubling is tricky" becomes just a boogie man. What i have to watch for is that I started doubling on trombone a long time ago. Like 43 years back. Because of this my mind automatically perceives trombone playing as a snap. Which it is relatively speaking. Easy. The problem occurs when i rely only on these automatic and sub conscious habit/abilities to carry the day by themselves. Ie: i forget to practice doubling on the fly and all the inherent prior discipline which goes with it. So I may play a low brass gig with no practice on the bone in the recent weeks or months past. At that time I will expend the ability of my upper lip to recover from the common low brass instrument mouthpiece caused swelling and find myself SOL on the trumpet for up to five days following. So definitely consider doubling on low brass but just be advised that it is a very practice dependent form of technique. The same is also true for mouthpiece switching (from large to small) on the Trumpet, Flugel Horn and French Horn but the effects are less noticeable. In fact if you plan to use more than one trumpet mouthpiece on a gig (say shallow for lead and deep for classical) it might be a good idea to learn a little low brass doubling. To deal with the altered chop conditions. Once low brass doubling becomes ingrained and a steady performance diet the chops begin to feel armor plated. Nice feeling. It isn't so odd that James Morrison and Maynard Ferguson double(d). They play so much that their chops became steel.