Imagine I was just handed a piece of sheet music, and I had however long I'd want to look over it before I actually started playing. What exactly would I look over so I'd sight read better?
There are two interesting levels:
- What is this sort of music? (Try to hear the melody, the rhythm, where are the notes one should emphasize, ...)
- Where are problematic notation parts? (Unexpected key changes, modulations, accidentals, clef changes, what are the notes with many ledger lines)
When looking at the sheet, try to imagine how to finger / pluck / whatever appropriate for your instrument. Anything you prepare now, will work faster later.