1. Visit the website
Familiarize yourself with their procedures, hours, requirements, etc.

2. E-mail the librarian with your intentions and questions
They are there to help you!

3. If possible, make an appointment
Some will want to take your registration and issue you a special credential; you may need to budget extra time for this. If notified in advance, many libraries / archives will be have your materials waiting for you upon your arrival. This saves you valuable waiting time. In some libraries, oftentimes the wait for materials can stretch into hours. Also, be aware of cut-off times for requesting materials before closing.

4. Make sure you understand their policies about copies
Some will allow you to scan and/or photograph documents. Almost all have some restrictions on their use. Make sure your understand permissions.

5. Bring what you need for your research
ID, pencils, paper, any notes. You may be required to park everything other than blank paper and a pencil in a locker; nonetheless, your notes may be helpful to have nearby. If permitted, bring a scanner, camera and tripod. The tripod is especially helpful. Don't forget any and all cords and extra batteries. Some also permit you to bring in your laptop. Some ibraries will give you a key for a locker, others send you to coin-operated lockers, so make sure you also have a supply of quarters. Some take dollars bills and/or credit cards.

6. Wear comfortable clothing
In some archives you cannot bring in a coat; during the summer months sometimes the air-conditioning can be powerful, so, whatever the season, you might consider wearing a long-sleeved shirt. Tight clothing can become uncomfortable after long periods seated at a desk.

7. Pack a power snack and a bottle of water
You won't be able to bring these inside, however, you can leave them in your car or the locker the library provides for you. If you are on a tight schedule, it's a shame to spend an hour of valuable research time hunting down a mediocre cafeteria or vending machine lunch. On the other hand, it's no fun to work on an empty stomach. Better to eat a healthy snack, then, after you've finished with your research, and the library has closed, go for a proper meal. Make sure the snack has both carbohydrates and protein. My favorite is a Lara Bar (nuts, dates, etc.).

8. Always, always ask the librarian
That's what they're there for! Librarians are a species of angel. I am not kidding. Always say thank you, so they will keep on helping you!

9. Respect their rules
If you don't, the angels may bar the doors.

10. Scrupulously note sources
Always make sure you note what you need in order to retrace your path to a source. This is key for footnotes.

11. Scrupulously note verbatim quotes
This will help you avoid unintentional plagarism, something that has embarrassed many an historian, alas. When quoting directly, I always use quotation marks. When noting something in my own words I often insert in brackets [mine].

12. Allow ample time in your schedule, the following day, if possible, for follow-up processing, filing, and etc.
The key to making the most out of your research is to keep it organized. Label, label, label and file, file, file. Big unprocessed piles are invitations to procrastination and confusion.