I have always wanted to be able to type the ⌘ symbol for various reasons, including writing tutorials, but I have not know how to access it through my keyboard. A few, general, related notes:

1. There is a nice wright up including some history on the Command Key, ⌘ on wikipedia.
2. How Apple Keyboards Lost a Logo and Windows PCs Gained One
3. PopChar is an application which helps users find obscure characters.
   
   

   PopChar is a utility for helping users find the Characters they are looking for

   This functionality is built in to OS X with Character Viewer, though it is likely that PopChar extends the user experience in some way.
   
   

   OS X Character Viewer

   

   Shift Key in Character Viewer

4. This discussion on the Apple Forums talks about a way to put these symbols in Pages’ auto correction so that Pages will auto correct a set of characters typed to the symbol desired. I have seen this used in MS Word too.
5. A table of Unicode characters corresponding to Macintosh keyboard symbols, as they commonly appear in menus.

The Next two Links are more detailed but like the above.

6. Special Key Symbols
7. Apple Keyboard Symbols

Marginally relevant:

8. Multi-stroke Key Bindings
9. Keystroke mapping explained by SIL’s NRSI.

It is unicode point 2318 (the html hex code is ⌘ ) and so you can find it in the character palette under:

• Code Tables>Unicode>2300>2318

or you can go into

• All Characters>Symbols>Technical Symbols

.

There are a few other ways to get at it, but that should do it for you.

On OS X, if you switch your keyboard to Unicode Hex Input, then holding down opt allows you to type the four digits for a unicode symbol and get the ⌘ (2318).

The Alt/Option Symbol has also been elusive. It can be fount at Unicode point 2325. U+2325.

Unicode and Hex Keyboard symbols
⌘ – ⌘ – ⌘ – the Command Key symbol
⌥ – ⌥ – ⌥ – the Option Key symbol
⇧ – ⇧ – ⇧ – the Shift Key (really just an outline up-arrow, not Mac-specific)

⇥ – ⇥ – ⇥ – the Tab Key symbol
⏎ – ⏎ – ⏎ – the Return Key symbol
⌫ – ⌫ – ⌫ – the Delete Key symbol