Very first tip is a basic tenet repeated throughout the guide; however, it is far too easy to become engrossed in your achievements (eg an awesome artifact, maximizing stats, a slightly better item, gaining a second blow, getting a speed bonus, etc) and lose it all due to something very avoidable had you been paying diligent attention.
While playing, always make sure to check for the basics – particularly monsters, stairs, and traps. Don’t lose a character, asserting that you can kill a “C” easily only to discover that a “wild dog” is a far cry from a “hellhound”.
Similarly, if you just got some nice artifacts / other equipment or achieved a significant accomplishment – save your game and take a break. You don’t have to continue to play to enjoy your past success, and when you return you will likely be more able to play diligently.
See a few uniques or other monsters that might possibly give you some trouble – maybe? Perhaps with only slight difficulty? See a vault with some cool treasure, but a few troublesome monsters / uniques? If you’re not highly confident (perhaps even 100%) that you can easily survive the level, you can leave on the spot. Just enter the level, immediately do your detection routine (monsters, stairs / traps, etc) assess the immediate threat, and use the stairs immediately if you want to. You can go up as readily as you came down, and you can go down as readily as you came up. Further, artifacts can be regenerated (unless you disabled that option) and as you dive, even better items will be available. So, if you skip out on levels just because you want to play it very conservatively, you will still find great items and better yet, you have a higher chance of survival.