I am not the worlds most advanced piano player, but my sight reading is very good.
A few tips on how I improved my sight reading:
1: funny enough this is not music related, but I started to learn to read blocks of texts. Take a big book, focus on one point and see how quickly and how much text you can read without moving your eyes. Do this every day half an hour and you will notify improvement also on sight reading music.
2: quickly browse through a music piece and highlight a few spots that seems very important for you with a colourmarker. It is easier to read from marked to marked piece and you have some guidance already.
3: hand/eye reaction is more important then you think. Do all kind of excercises to improve your hand/eye coordination. The better that is, the quicker you communicate between the different brainparts that are needed to do good sight reading.