Always have a goal and a "can-do" attitude. It sounds WAY cliché but its the truth. I know from experience. With the trombone I put my mind to accomplish goals that people found impossible, but I did it.

Also, just keep practicing and having fun. Make sure you never lose track of why you play trombone. Keep your sights on the stars and I'm sure you be happy with your playing.

I do have a few tips...
1) Be careful about playing advice from the internet. NOTHING replaces the advice of a competent teacher.
2) There are no shortcuts. Take the time to learn things the right way the first time around. Extra time spend working proper technique from the beginning will save you time spent learing it all over again. Strive for consistency.
3) Do a lot of listening to well respected bone players. Ask your teacher for recommendations. Know what a trombone is capable of , and know what you want to sound like.
4) Make sure your horn is clean and in good repair. Have an experienced player check your horn out. Your slide should problably move better than it does.
5) Practice does not necessarily make perfect. Perfect practice makes perfect. Learn HOW to practice.

Of course there is more. If you don't have a private teacher, get one.
Have fun.

make sure that you know how to hold the trombone properly.

Yes I know it sounds stupid but it can play a huge part in the sound and when your marching.

I'm curious why you say this? In my experience, marching band tends to require the player to hold the horn at a very unnatural angle.
I, for example, have a very large overbite, so I tend to hold my horn at a more downward angle. When we march, and a horn angle parallel to the deck is reqired, it usually makes it much harder for me to play, not easier.

I'd say as for holding...maybe this is what you meant, don't hold your horn at a extreme angle downwards maybe...I had that problem a while back where I'd hold the horn while standing so far downward that I'd actually bend over and play to the ground cutting making my air flow more difficult to be more open.
Is that something like your talking about?

Don't rush the upper register. If you try to climb the ladder too quickly you can start to lost flexibility. Happened to me for a while. Take it good and slow on the way up.