I've got the secret to improving your sight-reading and it worked for me....
Play every single piece ever composed for your instrument and nothing is left to sight read!!
Seriously though, there's many a true word said in jest. The more you play, the more different styles of music you play, the longer you've been playing, the more theory you understand, all adds up to improving your ability to interpret a piece of music and give a reasonable rendition of it on first attempt. Most people think that if they just play the notes, without any thought, something miraculous will happen...not so. If you can read the music in your head first i.e. the key signature, style, character, rhythm flow etc before you even start, then you will bring all of this to the sight reading and it will be all the better for it.
Sight reading takes years and years of practicing to improve. It is not just about reading the notes but also a wealth of other things. Before I even start to play I do the following first (in any particular order)
1)Look at the overall style of the piece...is it Baroque, Classical, Romantic, dissonant, popular, jazzy, rock, etc etc.
(Obviously, you cant do this without years and years of experience playing or studying all types of music, but try and see if you can get a rough idea)
2)Check the time and key signature and mentally feel the pulse that you want to play at. Once the key signature is in your head, apply this to the notes in the piece checking where the sharps/flats are. Also, is the piece major or minor? Look for the starting key/chord and the raised leading note in the Harmonic Minor scale e.g. G# in A minor. It is important to note if there are many or any accidentals in the piece too. This is also a clue to the style of the music..i.e. if the music has lots of accidentals, then it either modulates a lot, with slight dissonances or it is a dissonant piece of modern music.
3)Try and get a feel for the flow of the music, its rhythms by looking at the phrasing and note durations etc.
4)Look for any difficult bar/bars of music with regards to high/ low ledger notes and any tricky rhythms. Just quickly work out these few notes and visualise the rhythm. You usually get one or two tough bars like this in any piece.
5)Look at any dynamics and directions. I usually make a basic dynamic effort e.g. its loud at the beginning and soft at the end. Hopefully, I'll catch other gradations of music like crescendos and rits along the way...check these out first too if you have time.
6) Dont forget to check the tempo....is it adagio, allegro, moderato etc? If its a slow or medium pace, you will have more success at playing the piece as it finally should be played. If its Allegro, forget trying to play it at top speed as it will come out a mess. Try and go for a reasonably controlled, slower tempo, where you are in control.
7)Finally, take a deep breath and go for it, trying to remember all the details above. Dont worry about getting it note perfect, instead try and get the style of the music, a sketch of it, and do keep moving forward. Never, ever, go back to correct mistakes or get bogged down with one part. If you cant read it at first, just gloss over it and keep moving along.
(Imagine listening to your favourite pianist on CD and just having a slight scratch that just misses one minute part of the music and then continues nicely on its way as opposed to an annoying CD that gets stuck in a particular place for a long time before it continues. The first is much better than the second. Obviously, if the whole recording jumps and misses from start to finish, then expect a low mark!!
This might seem a lot of things to remember, but believe me, it does become more natural, the longer you've been playing. I can do more or less everything mentioned above quickly within the few seconds you have to study a piece you've never played. Sight reading does improve with experience and the longer you've been playing of course!!