The first thing I tell all of my style clients is that every well-dressed man needs a full-length mirror in a well-lit part of his house. Anything short of this is setting yourself up for failure.
Would you try to assemble furniture with your eyes closed? Of course not. So why would you try to assemble your look (your self-presentation for the day) under those same conditions?
This might seem like a no-brainer, but after updating so many wardrobes, I assure you that this needs to be said. My first assumption is that most men don't really care or worry about their appearance enough to justify giving this thought. Yet an equally likely culprit is that men are afraid of what they'll see.
With a mirror comes accountability. When you pass it by, you're reminded of how well or poorly you carry yourself. It's easier to pretend it doesn't matter/that you don't care when you don't have to look at the results of your work. Or of your neglect.
But if you are regularly faced with your reflection, you will be incentivized to make sure you like what you see. The mirror will motivate you to practice the skill of style, reward your success, and punish your neglect.
That's sort of a metaphor for introspection, but it'll literally do that as well. Better yet -- and in the same vein -- a full length mirror is SO helpful for shadowboxing. Being able to view your adjustments and mistakes in real time is a powerful supplement to mindful practice and habit building.
Practice makes progress and reflection breeds refinement.
(While this is true for both style and striking arts, a mirror won't do shit for your Jiu Jitsu. Sorry.)