Theres no shame in walking off the job if the boss refuses to do stuff the correct or safe way, ANSI Z133 is the standards book for tree work, if they refuse to acknowledge it, RUN DON'T WALK, other than that, what
@lone wolf said and you will be fine
show up early, stay off the phone, STAY HYDRATED, I often stop every 30 minutes and take a little break to have a bottle of water, I've had heatstroke once and came really close 3 times this year, it is NOT something you want, and can kill you without you even knowing you have it, if you ever feel lightheaded STOP AND DRINK, headache? STOP AND DRINK, colors and sound going away, you went too far, way too far, if its above 80 degrees expect to drink a full case of water each and every day, if your piss is anything but clear, start pounding water till its clear, and continue to drink water after work
don't drink energy drinks, and if you drink coffee limit yourself to one cup a day, and avoid crap like twinkies and donuts, tree work will possibly be the most work and exertion you have ever experienced, and it is not to be taken lightly
when feeding the chipper, stand to the side and feed one limb at a time, feeding multiple limbs is a quick way to chip yourself, also, gauntlet gloves are a no-go, I straight up banned leather gloves entirely on my jobs if we are chipping, they will not tear if they are snagged, cheap hardy cloth gloves are good, or just barehand everything
watch your steps for dog sh*t and trip hazards, keep the area behind the chipper clean, and don't be the first one to grab the rake or blower, as the new guy they are going to expect you to be doing the work they don't want, get to picking up sticks, and little stuff like offering to clean the truck windows while they are fueling up will go a LONG way
like rarefish said, if you hear the words "Headache" or "heads up" the only appropriate response is to get everyone out from under the dropzone and yell "ALL CLEAR" once everyone is in a safe spot
and, EVERYONE on a job has the right to issue a stop work order over any safety concerns, the rest of the crew may not agree, but if you see something unsafe, part of your job is to make sure it's rectified
I had a job earlier this year, total **** show from the beginning, subcontracting my company to another for a "1 day simple oak removal" that turned into 3 days of rigging, I got hit in the head because the owner of the other company pulled the limb I was lowering onto my lowering line locking it up, I had to go under to free it and a limb broke out and hit me, almost went to the hospital for it, the next day we are dropping the tree, rope tied to the spar, tying a rope to his truck to pull it, I refused to tie the rope to his ball hitch and told him I would be right back with my pintle hitch (which locks the rope in place, and is rated way higher), he told me no, I issued a stop work order until he agreed, my whole company got fired the next day because we refused to play his stupid game of "who gets to die today and who pays for their funeral?"
long winded post, I could go on and on, im willing to stop by if you have any more questions, im gonna stop here before I fill the thread with stuff nobody will read LOL