My sight reading isn't as good as it should be either, and whilst I would agree that plenty of regular practice is important, I think it is just as important to know how to improve the various different threads that together make a good sight reader.
Someone on the forum in another thread on sight reading recommended the book "Super Sight Reading Secrets" by Howard Richman, and I have been using this. There are certain stages of achievement that you must meet in pitch, rhythm and fingering before going on to the next steps, and each step focuses on one particular area before being combined with anything else. Keyboard orientation skills go along with visual perception skills and you must meet the requirements of both at each level before progressing to the next level.
The book is very flexible in that it can be used at any stage of learning, starting at the level you can manage now, but be warned that you have to be patient. Richman says that a thorough mastery of the core drills alone can take between three months and four years and you must always be patient and go at your own pace.
The reason Richman wrote this book after a lot of research is that he could "...play Chopin Etudes at the speed of light, but couldn't read musical comedy tunes at parties.
Embarrassing is an understatement ..." I can certainly relate to that, though I do admit that I can't play Chopin's Etudes at the speed of light either!
Although it is aimed primarily at keyboard players, you can adapt the ideas to ther instruments.
Anyway, I'm giving Richman's methods a serious go and would recommend this book providing you can keep going patiently.
Information about the book can be found at
Amazon. Note that it takes about two weeks to arrive from the States.