The thing that really helped to improve my site reading was to start transcribing drum parts. It can help get you to the point where you can read most music like the newspaper.

I had a student at the Armed Forces School of Music who was just a monster reader. So, one lesson I took out the hardest concert and rudimental stuff I could find. We site read it together. We were blazing through this one chart when he started doing the unthinkable. He started carrying on a conversation with me while we were both playing though the chart! "So, how's your day been so far..." etc. etc. That put me over the edge.

There are readers and then there are READERS.