OK, I'll add my two cents.
First, switch from ITYTD to HNTR. Playing too long on the easiest difficulty is only good to give you bad habits, like relying on the double healing, ammo and powerup duration. Getting rid of these bad habits can prove a harder challenge then the game itself.
Play patiently. When presented with a tricky or dangerous situation, take your time to consider your equipment and the level layout, and how it can serve you best.
Remember that there are times when discretion is the better part of valor. There's no need for a 100% kill rate to win the game, besides bragging rights.
Use cornershooting. Use it whenever the level layout allows for this.
Try to find sweet spots which allows enemies to enter your line of fire without a chance to retaliate.
Do not charge into unexplored areas. When moving to those areas, try to move in such a way that reveals as little as possible in each step - that should give less monsters an opportunity to take a shot at you the moment you enter their line of sight.
Learn which armors are better to use against different enemies. Green gives bullet and shrapnel resistance, and is good against former. Blue gives plasma resistance, and it is the best choice against hell knight (not barons!) and arachnotrons. Red grants fire resistance and is the best choice against revenants, mancubi and archviles (and in this case, the fireproof armor assembly boosts its efficiency even further.
Use those medkits and phases when you need them. They are no good in the mortem file.
Find a build that suits you. If you feel frail, try a Army of the Dead Marine build (Marines have inherent extra 20% max health and 50% powerup duration, and Army of the Dead is awesome against tougher and more armored enemies). Leave the trickier builds like Sharpshooter for the time you know the game better (and let me tell you - Sharpshooter is a pain to start, but incredibly powerful once it gets rolling).
Learn about special levels before you enter them.
Don't hesitate to consult the
DoomRL Wiki - you can learn lots about enemies, special levels and equipment there.
Good luck :).
Drop that damn "null pointer" or "blaster" - you are never use them! And combat pistol too!
* ZicherCZ the Sharpshooter prepares the P3B2 combat pistol and calmly aims at Equality ...