The main Family History Library in Salt Lake City is like no other library I have ever been in. It is definitely a paradise for genealogists. Before you take your trip, you should try to organize and develop a plan. Make a list of the people whose documents you will be searching for and get the FHL microfilm numbers. Bring a USB drive with you.
I knew that most of what I wanted to find were records of people in New York City. Although there were a few very specific people for whom I was searching, most of what I was looking for was documentation for people with a few very specific surnames. I anticipated that there would be fewer than 100 records for each vital record category – birth, marriage & deaths.
The first thing I did was to go to the Steve Morse website stevemorse.org, choose vital records, and then open the search for birth records. I put in the surname for which I was looking and when the search results came up, I copied the whole list and pasted it into a spreadsheet to which I could easily refer.
Now I had a working list of first and last names, dates, certificate numbers and microfilm numbers. I could remove any records from this list that I knew I was not interested in.
The 2nd floor of the FHL had all the microfilms I needed. There are huge banks of cabinets all clearly marked holding 10s of thousands (maybe 100s) of microfilms. I pulled out the first 4 on my list and went to one of the microfilm readers (proscan) that had the ability to allow me to save to a usb drive or to print. There is a 30 minute limit on these machines, but if there is no line, you can use them for longer periods.
Good luck with your research!