1. sign into Dashboard
2. refer to LIST or to this LIST2 or for HTML entities refer to LIST3
3. type the codes for the characters where you want them.
4. click Preview to make sure the characters appear as you want them to be
5. if so, save.
For example, if you want to display <a href="X">A</a>, you will have to type the following into your post editor:

and if you want to display ♠ ♣ ♥ you will have to type the following into your post editor:

Note that it is possible to use these HTML codes to display special characters in post titles as proven by a test - Test displaying special characters in post title.
Books from Amazon
Essential Blogging : Selecting and Using Weblog Tools
Blogging: Genius Stragies for Instant Web Content by Biz Stone, former Senior Specialist on the Google Blogger team