This is a followup to my post from January on typing special characters in OSX. Basically you enable the Keyboard & Character Viewer from System Preferences ▸ Language & Text ▸ Input Sources. Then you to to the Text tab and create a shortcut (usually I do a semicolon followed by a name or abbreviation). To enter the special character find it in the Character Viewer and double click. It’ll be typed. Now when you type your shortcut, say ;cmd, it’ll be replaced by ⌘.
It’s not as well known but you can do exactly the same thing in iOS. The only trickier part is getting the special characters since there’s no Character Viewer in iOS.
I usually just type them all in some text that is synced with iOS. Say in Pages, Number, Dropbox, Evernote or the like. Once you have them in iOS you just copy each one and make a shortcut. Shortcuts are at Settings ▸ General ▸ Keyboard ▸ Shortcuts. The screen may look slightly different from this one depending upon how many shortcuts you have. (Apple only gives shortcuts their own screen when you have more than five of them)
This tip is also nice for typing names that have accents in them. For instance I created a shortcut for Noël so that the accented e would type right even on my iPhone. It’s also quite useful for typing Greek characters if you converse about mathematics or physics via email, chat, ADN, Twitter or the like.