As of NetHack 3.6.0, the #adjust command can take a count, specifying how many items to move from one slot to another. (For example, to split off one half of a - 10 darts, you can #adjust 5a to b.)
As of NetHack 3.6.1, items/stacks that have been split with the #adjust command will not automatically merge with each other unless dropped and picked up, or stashed and removed from a container. This removes the need to #name individual items to prevent them from stacking.
If you have two or more stacks of identical items, you can #adjust one of these stacks to its own slot, and they will all be merged into a single stack. (This is useful when #dipping identical items in holy water: first merge the stacks, then dip the stack to bless them all at once.)
Many players find it useful to #adjust certain useful items to the same slot across games. As an example, consistently having your key in the k slot might save you the annoyance of having to look at your inventory every time you wish to unlock a door. It's also a good idea to readjust your inventory slots so as to prevent typos from killing you: keeping a food ration on y might be lethal if you try to eat it while standing above a cockatrice corpse.
The + key can be used to reorder spells.