Store your bicycle outdoors.
Outdoor storage, even in a so-called dry climate is bad for your bicycle. Moisture, just a small amount of rain, can and will eventually cause corrosion. Cables, shifters and derailleurs are hard hit by this kind of storage. Steel cables can quickly corrode, then seize up inside their housings and stop functioning all together. Internal parts on shifters can also be effected.
Wash your bicycle with water.
This common practice is often used with no prior thoughts. Again moisture is not a friend to your bicycle. Cables and shifters, even chains will begin to rust when exposed to any kind of moisture. If you want to clean your bike, first spray it with WD-40, then wipe it off with a clean cloth. Avoid using WD-40 on your chain, it can wash away internal lubricants.
Riding off a curb.
Kids do this all the time, gotta be fun, right? This sudden impact on the wheels and spokes can, and often will cause major damage. Bent wheels, spokes and even damaged forks will often be the result of this poor practice.
Transporting Your bicycle.
Improperly or over aggressively securing your bike for transport can cause frame and wheel damage. For extended trips through bad and wet weather the moisture get find its way into cables, seat posts and cranks, allowing rust and corrosion. Improperly strapping or tying down the bike through wheels can bend the wheels and damage the spokes.
Lack of use.
Perhaps the least evasive but still bad for your bicycle is lack of use. Over time, tubes will lose air, tires will deteriorate, then rust and corrosion will set in on vital steel parts such as cables and chains. Bicycles were built to be used.