For me, the answers are (1) Hiring a teacher, (2) Consciously looking ahead and (3) attending to intervals instead of notes.
I've spent the last six years working on my sight-reading (usually 1-3 hours per day) and I wrote a book about my experiences (see my signature). Here's a short summary from one of the last chapters:
Sight-Reading Dos and Don'ts
On the title page, I suggested that you “Follow my quest for sight-reading nirvana, and learn what you should and shouldn't do.” So, even though I never reached sight-reading nirvana, here are some sight-reading dos and don'ts that I've discovered:
● Do hire a teacher who is good at sight-reading, and good at teaching. If I'd done that on my first day, I think I would have doubled my first year progress (woulda, shoulda, coulda, huh?).
● Don't assume that if you just sight-read a lot, you will get better. This may be true for some people, but you'll probably get better faster if you practice the right way.
● Do learn to pay attention to intervals rather than notes. It's easier said than done, but worth the effort.
● Don't look at your hands.
● Do endeavor to look ahead as you read.
● Don't try to look way ahead – a note or two ahead may be all you need.
● Do be annoyed that the positions on the treble and bass clef represent different notes, but deal with it because it's not going to change.
● Do play at a steady pace and don't stop to fix mistakes. Playing along with someone else is a great way to force you to keep going.
● Do practice playing with your eyes closed. It's challenging and fun, and helps you find keys without looking.
● Do read through hymnals, but realize that you also need to practice with other types of music.
● Don't assume that one two-hour session of sight-reading is as good as four half-hour sessions.
● Do make a recording of your sight-reading when you start out so that you will be able hear your progress as you go along.
● Do leave stuff out. That is, instead of slowing down, leave out some harmony notes, for example. This is a skill that should be practiced.
● Do enjoy your playing rather than just seeing it as a means to an end.