Question: I am a Trinity grade 5 pianist and have been playing piano for 2-3 years(earlier played keyboard).

I find very difficult to sight read. I usually play the passage once very carefully and memorize it well so I don't have look at the music while playing. I play while continuously looking down at the keys and find it difficult to lift my head up and register the notes and then play. I know all the notes on the staff, but I can't register them in time to play them on the spot.

What should I do to get better at sight reading?

Answer: 
I know this is an old thread. But just do a lot of sight reading every day and constantly add unfamiliar pieces - stacks of sheet music that have nothing to do with the pieces you’re practicing. It’s actually a lot of fun. Pretty soon you’ll notice yourself instinctively able to find a lot of keys without looking. Also do scales without looking. You can isolate larger jumps and do them repeatedly without looking until that jump is easy. Soon you will notice you can play a lot faster if you don’t have to take your eyes away from the page, which makes you lose your place in the music. I also found Hanson exercises great for this if you play them just slow enough that you can take the time to recognize each note and keep your eyes always on the page.
I'm going to approach this answer from a different perspective, my own. For years I had trouble sight reading because it took more time to focus on the notes and be certain of their place on the stave. I have problems with my sight where I see double images when I look at things from certain angles so I can see double notes, double staves etc. when trying to sight read. Very confusing when sight reading. I had my eyes checked by several doctors with no solution, but I kept trying and finally found a doctor who knew the solution. He tested me, gave me a new prescription for glasses and now I don't see double when I'm wearing them. If your problem stems from eye problems, you'll need to get that fixed before you do anything else.