In general, just make sure you use lots of air.
On Euphonium you can slur--so don't tongue everything like you do on trombone.
Practice scales and technique with a metronome everyday, and never faster than you are able--It is very possible to learn to play with BAD technique.
Make sure your wrist is not bent and you use the fleshy part of your finger tips on the valve buttons, keep your fingers curved and relaxed.
Everything has to be clean and in control, then once you have mastered something at a particular tempo, then speed it up.
I recommedend these books Reginald H. Fink-Studies in Legato (Trombone/Euphonium) J.B. Arban (edited by Brian Bowman)-Arban's Complete Conservatory Method. Joannes Rochut-Melodius Etudes for Trombone Book I Dr. Charles Colin-Advanced lip Flexibilities (Vol. 1 is all you really need, it does come complete with all three volumes)
Re: euphonium i think that is how you spell it 14:18 on Saturday, January 5, 2008