Annie,
In addition to paying your dues with scale and interval practice, the other pieces of the puzzle related to great sight reading include immediate pattern identification (rhythm) and internal pulse.
I really like the book by Robert Starer called Rhythmic Training. I discovered this book many years after I graduated from college and worked through the first chapter (assuring that could “tap” through the first 15 exercises with no errors). When I found out that
Michael Sachs used this book regularly in college I decided that I would put this on my list of summer practice.
This book is like doing homework. If you spend time with the book (away from the horn, really concentrating on learning the material), the knowledge that you are putting in your head will simply be there for you to draw on when you are in a sight reading situation.
Here’s a post with some details on how I’ve used the book in the past.
Good luck!