I'm about to say some extremely geeky stuff. I'm a former English teacher. You've been warned.
When it comes to punctuation, the more the better. That's right. Punctuation is very good. It makes copy easier to read. Contrary to what you may have been told, sprinkle those little marks into your copy with abandon.
- Lots of periods means you're using short sentences. Very good. Very readable.
- Informal marks -- dashes and ellipses -- can bring both clarity and forward motion to your copy ...
- Commas are your friend. They open up space in the sentence, making it less dense. By all means, use series commas. They eliminate certain kinds of ambiguity, such as this famous but apparently apocryphal example of a hapless libertarian author whose book was Dedicated to my parents, Ayn Rand and God.
- Exclamation marks are great too! But they lose power if you use them all the time! And multiple exclamation marks can trigger spam filters in email!!!
- The only toxic punctuation mark, in my book, is the semicolon; it signifies a stand-offish erudition; most people don't know how to use or interpret semicolons. Just use a period and start a new sentence.