This reminds me of, on Project Runway, when they advise the designers to make sure their pieces look cohesive and part of a 'collection'. Jewel tones, A-lined or layers, and statement accessories. The end.
TIP: Try to sum up your style goals in 3 words -- That will provide focus. The more you focus what you're trying to express with your wardrobe the easier it is to tailor your closet to that goal.
Fashion rules are never hard and fast. However, when you're trying to talk yourself out of an impulse purchase or pressuring yourself to adopt some trendy style (pour one out for crop tops) go back to your own personal fashion foundation. It's more important to feel confident in what you're wearing than to stick with passing fads. Sure flares are coming back in style, and I may even wear some, but if I'm feeling out of sorts you know it's just gonna be leggings and that brown and black shirt and I'm just fine with that.