Master the Fundamentals of Visual Design
Good cover designs follow basic design principles even when they appear to break all the rules. Familiarity with these basics gives you the foundation for making good art whether you're hiring an in-house artist or doing it yourself.
Composition is the cornerstone of any good visual design. The rule of thirds, leading lines, and balanced elements all draw the viewer's eye and create visual interest. But purposefully breaking these rules every now and then will make an even bigger impact - just make sure it's on purpose and not by accident.
Color psychology is a very significant factor in album cover design. Warm colors like oranges and reds convey energy and passion, while cool greens and blues indicate calm or melancholy. Contrast high and loud creates drama and grabs attention, and subtle tonal variations are sophisticated and intimate.