There are many methods to protect historic properties. Here's a quick primer.
National Register of Historic Places: The National Register- contrary to popular belief- does not provide that much protection to historic houses. NR listed houses can be altered or demolished at will by their owners unless they are protected by one of the tools listed below. The only protection that NR listing provides is that if federal or state funds are involved in a project (such as widening a road) and NR properties will be affected, the project must be reviewed to see if there are ways to minimize or eliminate any negative impact.
State Register: Most states have state registers of historic properties. The protection that this provides varies by state. Most building codes, including ours in Massachusetts, exempt national or state listed properties from having to meet the building code in its entirety. For example, when renovating a state register-listed property, the inspector may waive the requirement to replace a non-compliant set of stairs.
The other benefit of being listed on either the National or a state register is that the property is eligible for historic tax credits.
Local Historic Districts (LHDs) are one of the most effective tools used to preserve the appearance of historic buildings and landscapes. Massachusetts and most other states have enabling legislation allowing local communities to create LHDs. Typically if someone wants to renovate a property in an LHD, the LHD commission has the authority to review any proposed changes that would be visible from a public way. So if an owner wants to replace the original windows or build an addition, those changes would be subject to review prior to the owner receiving a building permit.
The tools listed above are all "public" tools in that they involve governmental bodies. One private tool is the easement or deed restriction. This means that a legal interest in the property is donated to either a private preservation organization or a public entity. This easement requires that the present owner and all future owners protect the property as per the terms of the easement.
Many communities also have demolition delay by-laws which allow the local historic commission to review any building permit applications involving demolition of historic properties. While these by-laws can't prevent demolition (courts have determined that this would constitute a "taking") they can be used to find other solutions. Sometimes a buyer steps forward who is willing to move the building. Sometimes the owner is convinced that demolition is not the best choice.
For more information on any of these tools check with the National Truse for Historic Preservation or your state historic preservation office.