Adjust
#adjust is the extended command to change the letter that represents an item in your inventory.
Consolidating Inventory
A key function of the adjust command that is not immediately obvious is to force NetHack to consolidate identical stackable items into a single stack in the inventory. For example, if you are dipping potions with the intent of making holy water, you may start with ten different potions, each in its own inventory slot. Dipping them one at a time until they are fully diluted will leave you with ten separate potions of water, each occupying its own slot in your inventory. This is not a huge problem if you are using a water prayer at an altar, but is no good at all if you want to dip the diluted potions into a single potion of holy water that you already have. The answer is to #adjust one of the potions into its own inventory slot (b to b or whatever), which will cause Nethack to merge the other nine potions into a single stack.
Strategy
It is often useful to give commonly used items a letter that you can use each time you play the game. Here are some suggested letters for common items:
a - Primary Weapon - Your primary weapon always starts out being a. When you get a better primary weapon, adjust it to being "a" so you know it is now your primary one.
b - Secondary Weapon - Similar to using "a" as your primary weapon, most adventurers who start with a secondary weapon, will have it labeled b.
c - Primary Ranged Weapon - As an alternative, you might want to use "Q", which is the same letter as the command for placing an item in your quiver.
k - Key or Lock pick - As an alternative use "K", because "k" is also the command for kick, and if the player mistypes applying a key, they may accidentally kick a door instead.
p - Pick axe or other digging tool
u - Unicorn horn
Y - Stethoscope - "Y" looks like the item in question.
Another approach is to adjust key items to the inventory letter corresponding to the related NetHack command. For example, a Valkyrie carrying Mjollnir might put that weapon in the t slot, so throwing it is done by pressing t twice.
Consistency is probably more important than the details of any particular system, and it is all a matter of player preference.
Finally, it is useful to keep dangerous items out of the y slot to avoid YASDs. Nothing is worse than accidentally eating a cockatrice corpse due to mistyping.
