I looked back over my shoulder and kept watching.

Literal

I [topic-は] shoulder-beyond-[manner-に] turned-back-[sequential-て] continuously watched.

肩越しに (かたごしに, 'over one's shoulder') — 肩 ('shoulder') + 越し ('beyond, over') + に (manner). 振り返る ('to turn around, look back'). ずっと ('continuously, the whole time') modifies 見た. A vivid physical description.