I am not very good at the trombone but i really need some help. I cant hit High E Flat on Bass Clef.(2 ledger lines above) Usually when i do, it sounds all ugly and i cant do it again later. I also have problems with High D, but i can hit it sometimes. My lips get very tired.

I got some tips from my friends saying i need to relax, tighten my stomach and lips. Please give me more tips, i really need to know how to get a perfect note. Thanks in Advance.