Stress:
Stress can be one of the major reasons for acne and breakout. Most of the time people under severe stress can have a lot of skin problems.

Leaving Makeup On Overnight:
I think every one of us has their nights when they are too tired to take off their makeup and we just can't be bothered to go to the washroom and cleanse off our face, instead we kick off our shoes and straight to bed. But the thing we fail to realize is that one night of laziness and we will be punished for a whole week or more of bad skin. So it is always advised to remove eye makeup and cleanse off your face to free the skin of all impurities before going to bed.

Excessive use of Face Primers:
This might be a surprise to a few of you and not really good news for the Face Primer loving people but the excessive use of face primers is not good for the skin. As face primers are basically barriers that fill in the pores and lock in moisture and sebum (facial oil) and prevent oily break outs on the skin. But they can also block the pores and bacteria can get trapped in causing inflammation. Plus the skin excretes oil and moisture for many reasons one of which is cleaning the pores but as it is locked in by the face primer the impurities can not be removed from the skin. So it is advised to avoid every day use of face primers.

Reaction to Certain Products:
Some times we don't realize that a certain makeup product might not be suiting our skin type or we might be allergic to some ingredient in a product which might be causing break outs. It could be anything from a foundation to a blush/bronzer etc. So if you feel you might be having acne due to some makeup product and you are not sure from which then try not using one of the products for a couple of days and then do the same other products and eventually you will know which product was causing the skin irritation.

Uncleaned Makeup Tools/ Brushes:
Cleaning and disinfecting your makeup tools and brushes is very crucial as they get contaminated with bacteria and using dirty brushes again on your skin can be very harmful as you are applying bacteria to your face along with the makeup. Cleaning brushes and tools regularly is very very important if you are also using the same tools on some one other than yourself to avoid contra-actions. The brushes used for liquid/cream/gel products are very important to wash and disinfect as liquid/gel/cream mediums are perfect conditions for the bacteria to grow.

Shared Makeup Products:
Blemishes and acne can also be caused due to sharing makeup products like stick foundations, cream blushes, makeup tools etc. So always disinfect your products. And it is advised not to share products like mascaras as they are the best medium for bacterial growth and are not easily sterilized and can cause  condition from mild irritation of eyes to very swear conditions which can also lead to loss of eye sight. Mascaras should never be used over six months. Throw away the mascara after six months, doesn't matter if its finished or not, by that time it will be swarming with bacteria.

Not Disinfecting Products:
If you are using cream/liquid/gel products on some one other than yourself always make sure that you dont double dip your makeup tool in the product as it will transfer bacteria and germs to the product. Always use a Q-tip/ Palette knife or some thing to get some product out on to the back of your clean hand or a clean makeup mixing palette. Afterwards disinfect your makeup products even the powder products like eyeshadows and blushes with a spritz of rubbing alcohol to kill all the germs. Don't worry the alcohol dries quickly with out damaging the product. But make sure to avoid the contact of rubbing alcohol to your skin or eyes as it will dry your skin.

Unclean Hands:
Unclean hands is also a major reason for bad skin as throughout the day we touch many things which are carrying germs and bacteria and then when we rub or scratch or touch our face without washing our hands the bacteria is transferred to our face ready to cause inflammation, blemishes and acne. So make sure you wash your hands frequently during the day and never touch your face right after handling money as it is carrying the contamination from so many people.

If you are suffering from very bad acne try to include plenty of water in your diet. Multivitamin are also really good for healthy skin. If the condition gets really bad you need to consult a doctor for a superfacial ointment. But never use any ointments without the consultation of your doctor and never ever excessively use steroid containing ointments on your skin as their excessive use will whither your skin.

Hope this was helpful for you guys as I have spent a lot of time writing done my expert opinions :) .  Cheers!

Disclaimer : All the pictures in this post have been taken from the internet for the purpose of showing the examples of different looks.