Before considering a visit to NARA, go to the following web pages:
http://www.nara.gov/nara/nail.html
Try to use the search program to identify the record groups that you are interested in.
http://www.nara.gov/research/all/all.html
http://www.nara.gov/nara/dc/Archives2_directions.html

Get there early.  It takes a minimum of about 90 minutes to get any material to look at, and documents are only pulled once an hour.  Before entering the building, limit the amount of things you take in with you to a bare minimum.  They will not allow tablets, or paper, etc, although they will allow a few pieces of paper with notes on them, but they will have to stamp them before you will be allowed to enter. They even took away my hat. Hand scanners are not permitted, but a digital camera is allowed, and proved very useful to me.

(1) When you get there, first time users report to a receptionest room on the right as you enter.  Here, you will be told to sit down at a computer and enter personal information, and will have a picture taken for a photo ID that will permit you to enter the archives.

(2) If you are looking for WWII military records, you will probably be told to go to the 2nd floor.  You will have to report to the receptionist there and have your ID badge swiped.  You will then be directed to a glassed in room which was to your left when you entered the room.  This room has boxes of index material, which contains information on how to request documents.  If you are a first timer, this room will probably be confusing to you, so they will probably suggest that you talk to an archivist.  They will assign you a second badge, and escort you to another room on the second floor, where you will be assigned an archivist, who has a similar collection of index materials.  He will ask what you are looking for, and show you how to find the inforation.  I told him I was interested in 8th AAF , 41st combat bomb wing, and he pulled out a box from record group 18 (RG018) which specified 41st CBW on the outside.   This same info would have been available back at the glassed in room also.  This is where the research starts.
 

(3) Inside this box, were multiple sheets dealing with all sorts of things, from intelligence reports, flak reports, photos, and combat mission summaries, which is what I selected.  I took a picture of a few sample pages which were found in this folder.  The particular folder I looked in had listings of missions that I was interested in.  To request the actual information, you have to fill out a reference service slip.  This slip will list the box numbers that you are requesting, and the location in the archive where those boxes are located.  If the box numbers are contiguous, you can order multiple boxes on one slip, but if they are not, you need a separate slip for each box.  The box numbers come from the index sheets inside the folders, the other location info is found on the folder itself.

(4)  Turn your completed service slips in to the employee in the glass room.  The slips are collected on the half hour, and your documents are picked up around the corner from this room, generally in about an hour.  You can request something like 16 boxes of documents, which is basically whatever they can fit on one rolling cart.

(5)  Take your boxes to an available table, and do your research.  If you need to use their copying facilities (regular Xerox is $0.10 per page... overhead scanner for large objects is $0.25 per page), you will need to purchase a copy card from a vending machine.  The copy machines don't take money themselves.   Some of the documents are very fragile and crumbly, so be careful with them. If you need to copy a page that has a security classification on it, you will need to pick up a de-classifification slip to copy with the document.  If you don't do this (we didn't), they will be annoyed with you, and will have to staple your copies together and declassify the whole collection.

(6)  After returning your documents to the delivery desk, before leaving the research area, you need to have your Xerox copies stamped so that they can be taken from the building.
 

The above is just the bare minimum instructions for using the facilities, but could save a little time getting started.