Hmmm I'm not a very good sight reader myself but better than many other people around me. One very useful tip that I used when I sight read is to count the beat aloud while playing like for crochets I would count: 1 2 3 4.
For quavers I would count: 1&2&3&4& ( I actually say out loud "and")
For semi quavers or dotted quavers: 1-&-2-&-3-&-4-&- (- is pronouce it as "the")
So by counting out loud, I generally able to place my notes on time without going off beat. It might be hard at first but if you practice counting often u won't need to count aloud and still able to play it rather smoothly.
Another thing about sight reading would be the fingering. Before u start anything, I suggest taking a pencil and writing down some fingering. Dont write for every single note lol that would spoil it's purpose. Write for the starting chord/notes and write down everytime a change of fingering is needed, eitherwise br strict with yourself and use all your fingers to play the neighbouring notes.
General rule: lowest notes for right hand gets 1 and highest gets 5.
For left hand lowest gets 5 and highest gets 1
Lastly about the tiresome accidentals...well I normally play the scale of the key of the piece I'm playing in order to familarise myself with the accidentals...but all I can say it is staying alert and constant reminder to yourself :)
Relax when sightreading and start slow.
Curve your fingers with the palm of your hand in a ball shape to keep your fingers and wrist relax. Well that's all I can remember about sight reading :) have fun!
