Eric has lots of good points, so I'll just elaborate on some of them...
Some clubs are very competitive and insist on going for the highest possible contest score. Some clubs (like ours), treats Field Day as a fun thing...
This is very important - you have to gauge the expectations the club
has with regards to Field Day and what it is like. It gets particularly
messy then you have a mix of expectations: some hams focusing on
making as many contacts as possible.
When Field Day is a more casual social event, the focus may be more
on food and sitting around shooting the breeze, with a bit of radio
thrown in.
For competitive types, the site selection, antennas, stations, band
selection, scheduling, etc. may all need to be optimized. And you
will have a lot more log sheets to deal with, although it probably
will all be computerized with a LAN connecting the stations.
For a casual operation, one thing I've found helpful is to set some
sort of a goal, like making 100 contacts, or seeing how many states
you can work.
1. Site selection: This is absolutely critical. Be mindful of terrain...
This also depends on where you are in the country.
If you are East of the Mississippi, you'll probably have lots of
stations at relatively short distances, so height (to cover longer
distances on HF) won't be as much of an issue.
Here in Oregon, the bulk of the stations are further away. Sure,
we get a lot of stations from BC, WA, and California, and out to
Colorado and Texas, but, if we are being more than casual, we
need to make more contacts in the 1000 to 2500 mile range, which
will require lower angles of radiation (at least for horizontal
polarization). To facilitate that, I look for a spot on a hill that
slopes down to the East, and put the antennas at the edge of
the slope, or even a little ways down it.
Operating from Alaska, Hawaii, Guam, or other more distant
location requires a lot more care in site selection and planning.
Oh, and pay attention to poison oak/ivy, snakes, and other hazards.
3. Plan for logging options: ...
Also, plan to spend some time training your operators how to
log. I've seen lots of paper logs with illegible callsigns, no
indicator of who the operator was, incomplete sections ("NY"
isn't good enough), data entered in the wrong columns, etc.
If you have a new operator, have someone help them with
the logging.
For computer systems, make sure all operators know how to
use it, how to handle problems, and what the backup system
is when it crashes.
One approach we have used is to have a folder with log sheets
for each band/mode: that helps to prevent all your stations
switching to 20m SSB when that is the only band open. If
there are more physical stations set up than you are planning
for your entry class, you can use some sort of tokens to limit
the number of simultaneous stations in use. (Yes, all of these
have been problems at one time or another.)
4. Station Equipment: If your club has a trailer full of gear they use for Field Day, you'll just use that. Most don't. In our case, everyone who plans to operate brings their own gear, including power source, lighting, shelter, and antennas.
One good solution here is to decide what stations you want
to have (even if they aren't all on the air at the same time)
and appoint a Captain for each, who is in charge of recruiting
equipment, planning antennas, shelters, etc.
Visit the site ahead of time to see where the antennas can go,
and what sort of trees or other supports are available. Then
decide ahead of time which station and antennas goes where.
Otherwise you can have multiple stations expecting to use the
same tree as a support.
I've been at sites where everyone brings their own equipment,
sets it up in their own RV, and operates it by themselves: that
results in a lot of duplication of effort and doesn't give other
operators as much experience from watching. (But some folks
are more comfortable that way.)
There are two general ways that operating is divided up between
stations: by band or by mode. Dividing by band requires fewer
antennas - just one for each band (or a multiband antenna for
each station). For 3 stations, we typically would have one on
20m the whole time, one on 80/15, and one on 40/10m, and
the mode would be up to the individual operator.
If you have CW operators, it may work better to dedicate one
station to CW, and then divide up the other bands among the
SSB stations. Same with digital. That means multiple antennas
for each band (plus the GOTA station, of course). The Station
Captains need to know what bands / modes their station will
be assigned to plan antennas and equipment.
Also, make sure that the radios used "play well with others",
both in being less susceptible to overload, and not generating
phase noise that wipes out multiple bands.
5. Power: If you use a gas generator, ... test it in advance...
Specifically test it for RF noise.
One Field Day we went through about 5 generators before
we found one that would run reliably and not wipe out
the HF bands with noise. We had a couple stations running
on a small Honda, but the rest of the stations missed about
4 hours of operating.