Be observant when ppl show down.
Make notes.
Look for certain lines, when opponent has the goods. Do they donk sets, play call-raise or call-call-raise?
Learn to read your HUD well.

Play HU. Seriously. Verbalize your thoughts when u play, assign a range to him every hand. Try to figure out, WHY u´re giving him a certain range, what patterns actually made u think he´s holding this or that.
Leave table and look for a softer opponent if u can´t predict his ranges precisely after 45min