One of the most important places to start in archival preservation is security. It is very basic, but is an integral element of protecting collection. Security needs should address theft, natural disaster, fire, and other issues that could happen in your area.
This can be easily built into any space you are using for storage. Make sure that you keep archival materials in rooms that can be locked when they are not being monitored, and if that is not possible they should at least be in cabinets that can be locked. In most cases items will not be lost to theft but instead to use that is not tracked by the person in charge of the collection. If you would like to have your materials open to your community, it is best to put in place a system of checking items out or at least marking where they have gone that is strictly enforced.
Plan to install a fire suppression system of the highest quality that you can afford. All archival materials tend to be extremely flammable, particularly many media materials. At minimum, purchase a fire extinguisher and confirm that your smoke detectors are within the recommended age and have good batteries. Many archives have suppression systems that release water or foam, which is recommended, but it can be cost prohibitive in personal or small archive or in institutions with limited budgets.
It is very difficult to plan for the eventuality of all natural disasters, but make sure that you are considering what is most likely in your area. In the Midwest, it is best to keep materials in a tornado safe space, while on the coasts it is recommended to protect your materials from hurricane or earthquakes. Many of these things are difficult to fully protect against, but make the best decisions you can for your collections.
Flooding is prevalent in nearly all areas, and many archives have experienced water damage. Paper materials can be dried but media is likely to be irrevocably damaged or at least need extensive conservation. The best way to make sure that your materials are not damaged in a flood is to make sure that you are not storing anything on the floor. A shelf that keeps items at least a few inches off the floor will minimize damage if a few inches of water come into your space.
While it does not seem obvious, description is one of the best things you can do to protect your collection. If you know what you have, it is much easier to make sure that it is not lost or destroyed.