When starting out milling, when using a new mill and maybe (see below) before milling a new grain for the first time, trying  various gaps and milling regimes (more below) is well worth the little effort and time required.  The basics:   Measure and record the gap and regime, put a handful of you standard base malt in the hopper and mill it.  Adjust gap and/or regime, repeat and compare the crush of the two runs.  When starting out milling, spreading the milled malt out on black construction paper really helps assess and compare crushes.  After a bit of experience,  just collecting a bit of malt as it comes out of the mill is adequate.  Until you have even more experience,  don't simply look at the milled grains in the collection container for looks WILL be deceiving!.  Repeat the above until you are happy then mill, brew and change the gap and/or regime as needed based on how lautering went, efficiency and the brew itself.   After experience, setting up a mill and regime will become second nature and very simple.

Although my current mill is fairly radically different from store-bought homebrewing mills (it has rather smooth, large (4") dia. and narrow (2") concrete rollers) I keep the same setting for the 2-row pale "base malt" I use as well as all specialty malts.  I do a quick inspection of the first output each time I mill tho' to ensure nothing is radically amiss.  I seldom use 6-row as a base malt and know nothing of milling anything but barley based malts.

1.  Mill twice

I set the mill's gap a bit "too wide" and run the grain thru twice.  The first pass leaves about 20% (a rough guessimate) of the malt whole/unmilled and the second pass crushes most of this leaving maybe 2-3% unmilled.   It is said that two-pass milling roughly emulates how commercial 3 roller mills work.   This works great with my mill but YMMV since my mill is pretty weird.   Some folks narrow the gap for the second pass, but it gives too fine of a crush with my mill- YMMV.  If you don't narrow the gap for the second pass, much less effort on the mill's handle is required and one is tempted to turn it faster.  This results in a worse grind.  Again, YMMV, but you might want to watch for this when experimenting.

2.  Add Moisture to the Grains

Stirring in 1-2 tablespoons of water for each pound of malt one day before milling really reduces damage to the husks during milling and hence reduces the likelihood chance of a stuck mash.  The amount or water I add depends on what I guess the initial mositure content of the grain is- basically more in winter when it's dry in the house and less in summer.  I don't get real anal about precise amounts and notice little difference in how the grain mills.  Using a shallow container with a big surface area greatly eases stirring and aids in ensuring and even distribution of the moisture.  I use a plastic dish tray like those favored by most bus boys (err.. "bus persons").  It is much better than a plastic bucket.  After the water is sprinkled on, I pour the malt into a lidded plastic bucket until brew day.  This also increases the distribution of the moisture.   I've not yet let the watered grain rest for more than a day.  My SWAG is that if rested longer, the grain won't spoil but the mositure will migrate more to the guts of the grain leaving the husk drier and the guts maybe a bit too mushy.

The grind affects efficiency but, how much is debatable. I tend to agree with the often repeated advice "malt is cheap"!  I started out being greatly concerned about wringing the last possible smidgen of extract out of the malt but soon concluded that adding a bit more malt to compensate for the larger particles I prefer (and a few more unground grains...) and resulting lower efficiency was well worth the resulting reduction in stuck mashes.  The "ideal crush" for me is, in order of importance, each grain broken into 3-5 pieces (this a coarser mill than most folks seem to shoot for), fairly intact husks and minimal uncrushed grains.  Depending on your specific mashing setup and desires, your "ideal crush" will very likely differ since I use a RIMS for mashing.  They are more prone to stuck mashes than more conventional  mashing rigs so I favor intact husks and larger particles.  The trade off in my case is a worse case  efficiency drop of about 10% to 75% for my house ale.  With a finer grind and delaying starting to recirculate,  I can get 85% but the mashes are more prone to stick in the early stages of recirculation.  Again, this would not be a problem for mash rigs which do not constantly recirculate the wort and, especially, don't recirculate them during the first of the mash.