I'm not the best sight reader in the world,
my sight read also begginner-intermediate.
But my teacher recommend me this tips to improve sight reading:
1. Sight Singing, everyday
2. Major and Minor Scale Practice before sight read
3. Make up any notes if you mess up
4. Know the form : Sonata, Rondo, etc (for me it's really useful)
5. Practice (obviously), in any instrument is ok, but I prefer piano
6. Happy sight reading ;)