I used to be a terrible sightreader, one of the worst I knew. However, I think playing lead trumpet helps because everyone is counting on you to set the tone. You HAVE to play the rhythms correctly, and getting used to having that pressure on you, plus the practice, really gets things to improve.

For me, and I think many others, the hardest thing about sightreading is the urgency. You have to play a lot of stuff correctly with little time, because the music just keeps going. Sounds basic maybe, but I think that's really what makes a difference for many. Practicing sightreading with a metronome helps too. Reading without a metronome isn't as helpful because you can start slowing down, which makes it too easy. If you have that pulse, you can really see where you get messed up, and train yourself to maintain that pulse while reading rhythms. Also, if you lose track of where beat one is, you'll probably get lost. So keep an ear open for one.

I don't consider myself a great sightreader by any means, but I used to seriously be one of the WORST I'd ever known, and after doing these things, people actually tell me I read well on occasions. Hope I helped.