How on earth do you sight read both clefs when playing difficult pieces?! The standard advise I get is "Practise" but I am not sure what exactly to practice.

Are you supposed to read the clefs in a zig-zag pattern, starting at the treble clef, moving to the bass clef and back to the treble clef etc.? Or are you supposed to stare somewhere in the middle of both clefs so as to try to take in both "sides" of the music at the same time?

Kind of all of the above.

- Take in as much as you can of the music. If you can grasp scales figures and chords at a glance that helps. Nothing different from standard speed reading here. Knowing common patterns and grasping them at a glance really helps with sight reading. Practics is key here because it allows you to learn many different patterns and have them at your fingertips.
- If you do need to read across the staffs, be consistent. I think John v.d.Brook (one of the teachers on the forums) recommends reading consistently from bottom up on the staffs.
- It's also important to be able to read ahead. This ties in with the first point.

Rich

My teacher says start at the bass clef too.

Learning to recognize patterns, and knowing how different keys work, what the fingerings for different scales and chords are will all be very useful for your practice sessions.

Yeah you read from the bottom up.
There are a couple other sight reading posts you might want to check out.
It also helps to visualize/memorize the grand staff and what all the notes are.
Read then as intervals (the distance between one note to the next) as opposed to note by note.

And you with practice you WILL get much better and it will start to become more intuitive.