Try to recognize as many patterns as possible. Eg: recognize chords - a triad looks similar no matter where on the staff. So instead of reading C E G - you recognize the blob of notes as a C major triad.
Church hymns are good too. You can also just play the top and bottom notes for sight-reading practice too.
For me it's a fact that we do not read 2 staffs with 2 clefs, but a continous system with 10 lines. The clefs origin from the 5 line staff of "normal" instruments, but they do not have a real function in a piano score. Except when we play in two treble clefs or two bass clefs.So in the normal piano staff we have to know the position of the notes without thinking in 2 clefs. Just knowing which note (which key on the piano) is notated where in the piano staff. It's not difficult, but it needs training.