Read a variety of secondary sources before you starting working with any primary sources. The more you know about your topic the more productive your research will be. Serendipity happens. Create a list of keywords (people, places, organizations, events) and important dates. You can use these as access points.
Review the archives website or speak with a staff member before visiting.
Some collections may be stored off site and will require special arrangements.
Smaller archives may require appointments.
Confirm rules and regulations - copying, use of digital cameras, etc.
Consult all available online tools such as collection guides and digital repositories before your visit so you know in advance what you want to see. This will also facilitate copy requests when you are unable to visit an archive in person.
Take advantage of staff expertise and ask for advice on relevant/related resources.
Keep an open mind about what sources to consult. Not all materials have been preserved so on occasion you will need to use non-traditional resources such as photographs, posters, oral histories, etc. to fill in gaps. Historical newspapers can also be a helpful resource.