According to :
I'm using a Hilti drill to drill holes into concrete ceilings.
Drilling overhead with a heavy drill and big bit is making it
difficult to keep the hole straight. The Hilti is more than up to the
job ... the problem is when you're applying upward force, the natural
tendency is to pull the drill toward you or away from you, which makes
for a slanted or "crooked" hole. Any tips on how to keep the drill
straight so as to achieve the straightest possible hole?

Stick-on level vials are a good idea and might work well enough.

Depending on the drill and you, it might be possible to hold the
drill on your shoulder, and provide upwards force with your knees.
Should be much easier to keep straight with a little guidance
from a helper. Might need padding on your shoulder or against
your ears ;-)

Given "hilti" and "big bit" suggests that a guide (such as a
chunk of 4x4) might not be accurate enough.

To hold a guide such as this against the ceiling without timewasting
fuss (eg: drilling other holes), what you need is 2 jackposts of some
sort. Not the "real thing", but things more like telescoping cargo
bars. These are essentially just a spring-loaded shower curtain
rod with a locking mechanism.

If you got a long drill bit extension, it'd be a lot easier to keep
straight.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.