The absolute WORST thing you can do to your skin, regardless of skintype is expose it to the sun. UVA rays cause a lot of damage years down the road. This from an avid sun worshipper in my misspent youth. I'm quite religious about applying sunblock to my face, neck and hands every single day now.
The 2nd biggest skincare crime is poor cleansing habits. You must clean it in the morning and really take care to clean it in the evening to remove all traces of sunblock and makeup. This is one of the biggest contributors to clogged pores.
Poor nutritional intake. Not drinking enough water and liquids to keep your skin hydrated is another terrible crime. It's amazing how much your complexion will improve when you start drinking more water, green tea and adding omega fatty acids and fresh fruits and veggies to your diet.
Harsh cleansers, treatments and not enough topical moisture regardless of your skintype. Anything too harsh strips your skin of healing oils and can damage your acid mantel. To avoid this, be sure your cleansers and toners have the right PH balance. Healthy skin PH ranges between 5 and 7 PH.
Bacteria: Dirty pillowcases, washclothes, hands, makeup brushes, hair, keep it clean. Change pillowcases twice a week, use a different washcloth every single time, wash those handses before you touch your face, if you have too much product in your hair restrain it and clean your makeup brushes once a week at a minimum!
Lack of sleep and too much stress.
Too much makeup. If your skin needs improvement, focus on the skincare first rather than the makeup.
overzealous anything! This includes extractions, exfoliation, treatments. Keep everything in moderation.