How do you guys drill overhead all day without your arms falling off?

I did a stretch of commercial fitout work that was basically this, so here's what actually made the difference for me. First, the tool matters more than technique. If you're on a big hammer drill, get your hands on a proper SDS rotary hammer, the lighter class around two or three kilos. For small anchor holes they punch through concrete way faster, so each hole spends less time under load, and the drill itself is a lot easier to hold over your head. If you're already on an SDS, drop down a size. It's the seconds of resistance per hole that wreck your shoulders, not the total hole count.

Technique wise, stop death-gripping it. Loose hand, let the weight travel down through your arm bone, and stay directly under the drill instead of reaching off to one side, which is what ussually happens as the day wears on. Bracing it against your shoulder is a bad habit for exactly the reason you think, so replace it with something under the drill. Even a cut broom handle with a little cradle on top takes most of the weight off your arms. Better still, some guys build or buy a wheeled base with a vertical post and a drill mount, so you roll it along the floor and wind it up to height, kind of like the drywall lift idea mentioned above but purpose built. Crude but it turns an all-day arm workout into a light one. Also, that trigger finger aching at night isn't normal muscle tiredness, that's a nerve or tendon complaining. Rotate onto other tasks when you can, and if it keeps up past this job, get it looked at before it becomes permanent. And have a quiet word with whoever's running the job. Thousands of overhead holes is exactly the kind of task where they should be supplying better gear or splitting it across the crew, not watching one bloke grind himself into an injury because nobody asked.

Gotta push back on the 'tool matters more than technique' bit. I ran a smaller rotary hammer on a ceiling job a couple years back and honestly the little machine was worse for me. All that mass in a big drill soaks up vibration, the light one buzzes straight into your hands, so my wrists were cooked faster, not slower. What actually saved me was running a vacuum off a shroud on the drill. Holes stopped clogging and I quit re-drilling half of them, which cut way more time per hole than swapping machines ever did. What size holes are you drilling? If they're small the shroud setup might matter more than the tool swap.

Agree with the shroud and vacuum setup, that was the game changer on a soffit job I did a few years back. Running a hose up to the drill sounds awkward but I just taped it along the side rail of my scaffold frame and it never got in the way. The real benefit I found was the dust not packing the flutes, so the bit kept cutting at the same speed all day instead of slowing down as it clogged. Fewer seconds per hole adds up fast when you're doing hundreds of them.

One small thing that helped me on a run of overhead anchor holes for garage shelving: check your bit length. I started with a long bit for margin and it wobbled all over the place above my head, then a short bit in the same drill was noticeably steadier and faster because there's less of it flapping around up there. Batching helps more than you'd think too, snap a chalk line across the whole area, drill everthing in one pass along your travel direction, then go back and set the anchors, so you're not setting hardware with tired arms fifty times over. Are you marking each hole as you go or laying the whole grid out first?