Formal:
* Mgmt. (most common)
* Mgt.
Informal:
* Manage. (used as a verb, but can be used as a noun in some contexts)
* Manging. (also used as a verb, but can be used as a noun in some contexts)
The best short form to use depends on the context and your audience.
Example:
* Formal: "The mgmt. team will meet next week."
* Informal: "I need to talk to manage. about this."
Let me know if you have any other questions!