Take your time and only ever look at new pieces... As that's sight reading at its purest. 10 mins each day will make a lot of difference! Everyday!!!! you will make around 87 hours worth of pure sight reading... read music whilst listening, read music as if you are reading a book! sing in your head, sing out loud! sit next to a piano sing a few notes before you play the notes! Use the Niel tarlton books!! Immerse yourself... Most important thing is to play music. Learn a sonata i.e Vivaldi's Cello sonata no 3 - http://www.youtube.com/watch?v=4WyHvZyczCc
ABRSM do the complete cello sonata for db or you can probably find a pdf somewhere online alot cheaper!!