What are your best tried and true life hacks for the field?

propane/fuel squad stove, grounds, tabasco, poncho/liner, kabar, compass, wpn and dry ammo, d rings and para cord depending on environment, whatever you can fit in ass pack above wpn and clothes on back, seeds, nuts or jerky, trail mix

Eat matchstick heads to repel ticks
550 cord
100 mph tape
Poncho liner
Coffee grounds
Socks...take care of the dogs
Alcohol pens for waterproof writing on laminated maps
Promask as pillow

Throw away the field jacket. Useless.

Dummy-cord stuff so you don't lose it/leave it behind at 0200 in the woods. Buy a field jacket liner (if they still exist; failing that a sweater) and take it to the tailor shop to get it re-seamed and velcro-ed so you can put it on when you halt and take it off when you start moving again. (Further to that point, in cold weather, don't stop if don't have to, unless you have a really intense fondness for charlie-horses.) Open up your MREs and distribute the contents where you can get to it more easily (without having to get into your ruck.) Specific to release-road marches or any activity in which you anticipate running with your rucksack for a good distance: Put overlapping strips of hundred-mph tape across your lower back where the kidney pad rubs to avoid developing a gigantic sore that will otherwise greatly distress you when you find your t-shirt stuck to it.

I take my wet weather poncho if it isn't raining, and my heavy sleep system, wrap them together and then put it over my ruck. Pretty comfy pillow

We were tuning a ambush , training , I so hyed that I sound like a bull walking though the woods

My personal favorite additional packing items:
1. Solar charging panel
2. Woobie
3. 550 cord rope/donut
4. "Snake" knife (usually my 6" Gerber or Kabar)
5. Jolly Ranchers (keeps up the sugar levels that some MRE's just don't provide)
6. Instant coffee
7. Hand warmers (in the cold season)
8. Gatorades and Red Bull.
This is all in addition to the usual layout. Spare uniforms, e-tool, wet-weather gear, socks, boots, sleep system, etc.

Before they issued the current gen of ECWS layer 1 gear, I would wear the long lycra running tights under my trousers. Didn't add too much heat during the day, but helped to insulate at night.

It really all depends on the environment you're in or going to. A few of my "must haves" are a power strip, the old old brown wool sweater (the wool one and not the nylon one), and my own small med kit.
There is nothing like getting in the hooch to see that every outlet in the place is taken. Bring your own power strip and you have at least one outlet for you. Your outlet, your rules.
The old brown wool sweater is the best piece of snivel gear ever. Because it's wool it still insulates you if you're wet. It's soft enough to stuff into a sleeve and maekshift a pillow. It's a great cot liner too.
My med kit is not the save your life, patch you up from bleeding to death med kit. I'm talking creature comfort here. Tylenol, Ibuprofen, Benadryl, Zyrtec, nasal decongestant spray (the stuff from the dollar store is the best I've found), Chapstick, Tums, hand sanitizer, and a few bandaids because no medic ever has this stuff. It all fits in a ziploc sandwich bag.