Spoiler

The best thing to do (by far), is to learn the monsters' weaknesses.
With a good knowledge of which attacks and defenses to use against each
monster in the game, you can accomplish such tasks as beating the whole
fire tower with a lowly 99 HP's. So many deaths at the hands of ogre
magi... with a scroll of darkness sitting in ones pack. So many vortex
explosions... with a potion of invis that was never drunk. Almost all
deaths can be avoided, but don't feel bad if you forget something. Here
is a brief list of some simple strategies that I use, and you will of
course develope your own:

Mimics, vortices, eyes - Keep an eye out for purple items and don't
bump into them : ) All are weak against darkness and invis.

Tough humanoids like ogre magi, cavemen, and dopplegangers - Almost
all humanoids are weak against darkness and invis. This is especially
handy against Li-han-kay. Orcs and dark elves can see in the dark,
but not invis.

Griff Yard - Do this as early as you can, and get the reward as
quickly as possible. The blessed potions of extra healing will see
you through some tight spots. You need Dx = ~20, cold resistance, one
potion of holy water, a ring of stun resistance and a reasonably
powerful attack.

Water Temple - Darkness, as all inhabitants are blind in it. Just make
sure all monsters have an escape route while in darkness, because if
they panic and can't flee, they attack.

Fire temple - It's rare to die to anything but the ACW in here. Just
make sure you have a highly charged wand (or well trained spell) of
stunning. The ACW should be easy after that : )

Cat Lord - Don't kill cats. Yes it's worth the trouble : ) Lead him
to the previous level, and web him, then blind or confuse him with
potions.

Earth temple - Lead monsters up the stairs one-by-one. Alternatively,
use every fire ball blast you have and hope for the best : )

Mana temple - The only thing you really need for this is confusion
resistance, as much as possible. Wake the Archmage, and stand in the
hallway killing monsters, until you see him. Lead him around the
hallway, all the way outside of the temple entirely. Keep a 'buffer
monster' between you two all the time, to eleminate his bolt spells
(you can use a wand of monster creation if you need to create your
own 'meat shield'). When you get to the place you tunneled in at, kill
your 'meat shield' and let the body of the Archmage be a 'stopper' in
the mouth of your tunnel. This keeps away the interference. Just stand
out of a straight line with the Archmage (so he can't use death rays),
and kill him with (poisoned) missiles.

D:50 - Leave the center room alone, or at least leave it until the
end. Tunnel into the top and bottom rooms, use darkness against the
chaos goons, and invis against the ghosts. Teleport back and forth
to the top and bottom levers. Closing them will cause a single balor
to teleport to one and open it. You can kill them all one-by-one this
way, including Fisty.

Spoiler
Don't do Nuurag-Vaarn without death ray resistance. Gut is an absolute ADOM crack and will not make mistakes that lead to him taking a hit from a death ray, but very few can match his prowess, and it's always better to leave rooms for mistakes in ADOM.

D:50 sometimes has an undiggable left half. In this case you'll have to mow down some serious writhing masses of primal chaos. There's some tunnels that still take you to the lever rooms without you waking the guys at the gates. Also, if you use a wand of destruction after pulling each lever, you don't have to kill the balors, even though it's certainly lots of fun. Also, balors will open the levers usually only when you've already departed for the stairs.