These are the top 10 most damaging ways to treat you hair. They are in no particular order and most are regular ways of treating your hair so I don't expect anyone to avoid them all completely.
1. Styling with thermals. Hair dryers, hair straighteners, hot rollers, etc. We all love looking good and we all should if that what we want. But trying to minimise how often we style and the amount of heat we apply to our hair (especially when its not boy short and we aren't getting regular haircuts) is going to pay off when trying to keep your hair at its healthiest. Also using protective products before you apply thermals is going to assist in damage control. Try a leave in conditioner or a protective shine spray like Keratin Complex Thermo Shine.
2. Shampoo. While shampoo cleans your hair (and regularly cleaning your hair is obviously necessary) it also strips your hair of its natural oils. On going this can become extremely damaging. So the less you was your hair, the better. For more info on this topic read this.
3. Lightening your hair colour. Colouring your hair is not always damaging. Semi permanents can even add shine and smooth your hair temporarily. When you lighten your hair however, you always need to use an ammonia or other harsh chemical to penetrate the follicle and lift the hair colour. This is always going to damage the hair but to what degree is up to you. The less often you lighten your hair and make sure the chemicals you use aren't too strong, the less damaging. Not leaving the colour chemicals in your hair too long is also going to prevent over processing. For more info read this.
4. Chemical straighteners and perms. These are two kinds of chemical treatments I don't provide for my clients. Simply because I believe that the result they produce for the damage they cause is just not worth it. If I wouldn't do it in my hair, I wouldn't recommend it to anyone. The reason they are so damaging is that they change the natural form and shape of your hair permanently (although it does grows out). Please feel free to leave any comments or questions on this one as I'm not going into it much further in this post.
5. Extensions. There are so many different types of extensions. Some are far less damaging than others and to whom's hair they are damaging to all depends on who is doing it and the person getting it done's hair and also how well the extensions are taken care of. So if you are planning on getting extensions, shop around and go off recommendations. Pay a little extra for better quality hair. If you still aren't sure but want to try them, my favourite option is a clip in hair piece. You can ask your local hairdresser or wig maker to find you one that is the right length colour and thickness for your hair and show you how to clip it in and out so you can wear it whenever you like and it won't damage your hair at all.
6. Tying your hair tightly when you sleep. This puts a lot of strain on your follicles and eventually you will get some breakage. I recently had a new client who did this and was wondering why her hair always broke and started looking like a mullet. If you can't sleep with your hair out, then plat it loosely and loosely tie the end.
7. Dry shampoo. I love dry shampoo. You can get away with the oiliest mess of hair when you're in a rush. But using it regularly really dries out your hair and scalp and it can take a while to recover from. So use in emergencies only.
8. Brushing. If your hair is long, brushing regularly is a must and you should, once or twice a day. But always start from the ends and work your way up to the roots. it you start at the roots your going to end up with some serious breakage. Especially if you have product in your hair like hair-spray or dry shampoo. Combing your conditioner through in the shower or combing it after your hair is towel dried with a wide toothed comb is going to help you avoid knots generally also. For more info, read this.
9. Weather. The weather really only has a small impact on your hair. High, strong sun exposure can damage your burn and bleach and strip your hair somewhat, but its a slight gradual process. Wearing a hat and using a leave in treatment/moisturiser will create a barrier between your hair and the sun and protect it. You can even find some with UV protection, like sunscreen for your hair.
10. Stress & Diet. Kind of two things but they go hand in hand. The better you take care of yourself the more lovely you look. Stress can cause your hair to fall out and become quite brittle. As can getting sick and taking certain medication. Theres some extra info here on the subject. The more protein in and vitamins, minerals, oils and good fats you consume, the shinier, thicker and stronger your hair will be. Also taking supplements like Silica will help improve the strength and appearance of your hair. So don't stress, eat well and just take care of yourself.