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 ;)