I was tempted to post early on but as I kept reading, I thought how could I possibly add to the great advice given and pointed to in articles, tips, FAQs, essays.....
Well, I'm on this list to learn as well as to help when I can, so I read all that I could. Who doesn't want to sight read better. But the more I read the more I realized that maybe I still had something to add to this discussion. Three somethings, actually, so here goes:
1. R.W. Brown taught me something when I was about 10 or 11 that I think everyone should learn as early as possible. He said you should never have to sight read on the trumpet. The first thing you should do when you get a new piece of music either in a lesson, or practice, or audition or band rehearsal is to look through the entire piece. Notice all the key changes, dynamics and repeats, (DC, DS, Coda). Sing the piece to yourself as best you can looking for odd or difficult rhythms, large intervals, range and accidentals. Finger the notes on the horn if you have it in your hands or at least in the palm of your hand (air trumpet :). So by the time you put the horn to your chops to play, you're no longer "technically" sight reading. It's always worked for me.
2. Be very careful when sight reading a familiar piece. As Jeanne suggested, it's a great way to learn to sight read because it helps in the ear training aspect of the skill. However, always be sure you're playing what's written and not what you "think" is written. Just because it sounds OK doesn't mean you played it correctly. It's a difficult balance between playing by ear, recognizing and playing patterns and playing note for note what's on the page. Even the best players fall into the, "that's the way I've always heard it" trap. Concentration and always be critical.
3. Finally, here's one of the most important aspects of sight reading, IMO, that I don't think was mentioned in anything I read previously. One of the biggest problems people have when sight reading is getting lost. Forget about all your good practicing guidelines where you've trained yourself to stop on a mistake, work it out and play it again. Sorry! The ensemble won't wait. They won't even slow down. You have to learn to just plow through the mistakes. They WILL happen. They do on occasion for the best players. You can't let it throw you off. Learn to listen to the ensemble for an overall sense of time and where you are in the piece. Listen for 4 bar phrases. So when you miss that not quite chromatic run, or you hold a half note for 3 beats or that tricky delicate syncopated rhythm comes out sounding like a three legged elephant tap dancing, you can drop a beat or two or even a whole measure and pick things back up in time with the rest of the group. It doesn't matter how many "right" notes you played. If you get lost, the rest is just wrong.
Sorry that got a little long winded. I've been saving it up for a couple of days.