You can create new document classes and new packages. For instance, if
your memos must satisfy some local requirements, such as a
standard header for each page, then you could create a new class
smcmemo.cls and begin your documents with
\documentclass{smcmemo}.
What separates a package from a document class is that the commands in a
package are useful across classes while those in a document class are
specific to that class. Thus, a command to set page headers is for a
package while a command to make the page headers say Memo from the
SMC Math Department is for a class.
Inside of a class or package file you can use the at-sign @ as a
character in command names without having to surround the code
containing that command with \makeatletter and
\makeatother. See \makeatletter & \makeatother. This allow
you to create commands that users will not accidentally redefine.
Another technique is to preface class- or package-specific commands with
some string to prevent your class or package from interfering with
others. For instance, the class smcmemo might have commands
\smc@tolist, \smc@fromlist, etc.