How do I improve my embouchure?

I have played trombone for over 6 years. But I have a problem with my embouchure that prevents me from playing high into my range, and therefore i can't get first chair in either jazz band or concert band. The corners of my mouth don't close in on the mouth piece, the seal is not created. So i have to create the seal by pressing the mouth piece into my lips very hard, it can be very painful to play, especially play high. Any suggestions on how to improve?

"The corners of my mouth don't close in on the mouth piece" If this means you pinch the high notes, forego all ambition for a while and do long, low notes as an extended warm up, especially softly (playing piano is a *****). When you notice the fatigue, concentrate on NOT pinching. Ignore the air you lose, just keep your lips stretched and pulled downward.

[I hope I'm not offending anybody with that b-word yahoo seems to want to clean up, heh heh.]

There is nothing more effective than practicing day! If you already practice everyday you should practice for a longer period of time.... I know it may be a harsh reality, but its really the only thing that will help your embouchure...

I play trumpet and trombone. When I start on trombone I always warmup for at least 15 minutes on low F and low B flat.I play the B flat scale starting on low B flat. Doing that for a while generally loosens the chops and I gradually increase the range and have no trouble playing the high notes which must be played with the embouchure as relaxed when playing the low F. I have used the same mouth piece a Bach 12 C since I got the trombone in 1978. I hope you are not constantly changing your mouthpiece to improve your high register. I also play a song or several in the very low register first because that makes it easier to play the song in the higher registers.Listen and play along with trombone CDs by players like J> J> Johnson or whoever your favorite players may be.

Play LOTS of long tones! You will be surprised how much this will help. As you improve, focus on tightening your corners and putting air through the horn. Also, talk to your band director or lesson teacher about it, he/she may have some other suggestions.
Good Luck and God Bless