"44991" - I know that number well!
(slaps forehead) YEP! Get a digital caliper - the HF ones are surprisingly usable even at the discounted price.
Also another forehead-slapper: cutting fluid. You can use WD40 some, but you really need some cutting fluid. I'm not sure if compressor oil would work, but I don't see why not. If you don't mind the mess and the smell, used motor oil works surprisingly well, and in fact the reamer rental place recommends using motor oil to lubricate their reamers. There are good cutting oils like tap magic, but you'll pay a king's ransom for the quantity you'll need. Better to find a good, cheap alternative. I like the cheap lubricating fluid in the spray cans from harbor freight. In the blue label. HF has the silicone in yellow, and I can't remember exactly what it is called, but I think it is simply called lube oil. COmes with the little red straw. That works really well for me, when I remember to buy several cans' worth.
There is also the water-based cutting fluid which is what you see in those little ball-knuckle spouts on the CNC setups. That goes into a circulation bucket that has a filter or something. It is a complete setup. You could just let it pour out everywhere, but there is the mess to deal with. And you need to spray off everything with WD40 once you're done, for obvious reasons.
I've even used plain 'ol water before, in a water bottle. Just depends on what I'm doing.
It should go without saying, but good eye protection is doubly important with a mill. You'd be surprised how hot those little chips get, and when you have one land under your eye on that soft skin, you'll know why I recommend a full face shield. Those things are HOT - ask me how I know The cheap yardwork shield is actually my favorite setup for eye protection. Possibly some leather gloves, depending on how many chips go a'flying.
A good, cheap shop vac is handy to clean up most of the chips.
What else? Ditto on the comment about not trying to cut too much - read up on the cutting speeds and pay attention to when the motor starts to labor. I usually cut mild steel at just under the halfway mark on the speed dial, and harder steel at about 2/3 or so.
All on the "low" gear setting. For that matter, be sure the low/high gear changing lever is snug. When it pops out, you will think you broke a gear! Also, expect to break that gear a few times. I tend to punish my tools, but if you push it too much, it will break. I have only broken one other gear, but the 2 times (or is it 3?) that I have broken a gear, it has been the high/low gear. Fortunately, LMS has all the parts for sale, so you are only out about $15 and an afternoon while you remove and disassemble the gear head.
Also, buy that spray-on white lithium grease for the ways and gears. In the same place as the HF lubricating oil.
A good metal marking fluid such as dykem (is there any other?) will help you when marking a piece for the final cut. You scribe your line into the dykem so you know where the cutter needs to stop. You do have a scribe, don't you? HF sells some that are actually pretty good.
Oh almost forgot - an edge finder is indispensable when doing precision cuts, as is a deburring tool for holes.
Another one - GOOD LIGHTS! I have one of those cheap work lights and a 75-watt bulb right there at the ready. Helps when doing eyeball 1.0 measurements.
Lastly, a small pocket calculator helps if you are doing precision measurements. Read up on the way the dial reads, which is 1/16" per turn, or .0625", and you can easily convert that number to how many turns to cut X or Y amount. You'll figure it out.
I can't think of anything else. Don't be afraid to experiment, and remember the gears *and* cutters are all replaceable! Just don't forget a face shield.
http://www.onlinemetals.com is a good, but expensive, place to buy metals. I prefer aircraftspruce if I have a choice!