I travel to the Family History Library at least once a year--always taking a small group. I know others go when there are conferences in town, but I find that I get more research done when there's not a conference going on.
It makes it easier to concentrate.
Part of going is planning. A big part of going is planning.
My list of things to look for is already growing. Most of what I do while at the library is concentrated on "lookups" for myself--items that I can't get elsewhere easily or larger files or items I'd like to make digital copies of from microfilm. Often those items are things that in the day of photocopies I was too cheap to photocopy in their entirety. The digital images on the FamilySearch website are usually ok, but as some have pointed out, there are times a researcher can make better images onsite from the actual microfilm.
The card catalog is crucial to this proces. The Family History Library does not have everything. I cannot simply "go there" without planning and expect to find all that I need. There are significant materials that still have to be located in local and regional archives. I still have lingered on locating a deed from Winnebago County, Illinois, and a probate file from that county as well because the Family History Library does not have the item and I've not made the trip to the actual courthouse myself.
I usually keep a running database of the films I want to locate at while at the library. There are several fields as a part of this database, but the film number, floor, and items on the film are key. It may seem like the floor the film is on would be extraneous information. It is not when one is researching families in a variety of United States, Canada, and European locations. Running up and down stairs (I don't use the elevator) repeatedly is not a good use of my time. Sorting the file by floor and then by film number helps me make the most of my time in Salt Lake.
As I make my list, I often realize that the answers to some questions don't rest in Salt Lake.
That's good to know as well.
While the answers to many questions are at the Family History Library some aren't. It's always better to know that before you get there.