In order to sight-read you MUST know your key signatures. Without knowing these you cannot hope to even start sight-reading. Try to memorise the cycle of fifths at the top of the page, if you are good with words remember the anagrams, if you are great with numbers learn the mathematics behind the cycle of fifths and if your more of an artists try to memorise the pictures to help you learn each key signature.
Time signatures are really important. They help you count your way through the music. Why do you need to count when you sight-read? Rhythm is the most important part of music. It gives the notes purpose. Music without rhythm and pulse would be like you trying to walk without legs. The top number on the music is the amount of beats per bar. The bottom number is the type of beat per bar. For example: 4 4 means 4 crotchet beats per bar.
Look through the music, one hand at a time, practice any potentially problematic bits and make sure that you are confident with them before you start looking at the other hand. DO NOT RUSH THROUGH! You are not in an exam when you practice! Speed of learning comes with practice and takes time.
Bar by bar look through the music hands together. Make sure you look for patterns and to help you glide your way through the music.
6. Don't forget your dynamics and musicality.
Make sure you have looked at the dynamics, tempo markings and more. Music without musicality is just numbers. Examiners and audiences want to hear you play beautifully even if there are a few strange notes. Make the strange notes musical. Sight-reading is about how best you can perform at short notice, this includes musicality.