Step 1: Learn the piece.
Step 2: Line by line.
Step 3: Daily practice.
The next day/session of practice, repeat what you played the day before. Try it without the music first. If it's a little foggy, take a quick glance at the music, then try. Always review your previous practice session before you start something new.
When you have memorized the next 2 lines (or however many you have set for yourself), play it with the first 2, all without the music.