Subject
Remember that women looking over their emails will often sort their mail by subject, deleting uninteresting-looking messages without even bothering to read them. Make that subject line something she'll probably find interesting. It's best to make sure it relates somehow to what she'd posted earlier. Show her you really read her posts and what she had to say. Keep it short; make it witty when you can. Never poke fun until you've come to know her reasonably well (at least wait until you've written her for a while, and she's come to know when you're being sarcastic). In other words, use your head.
Length
Ever been in a conversation when someone's made a very good point, but then they begin rambling about things that have nothing to do with the topic at hand? How often have you wished the whole thing were on videotape, so you could just stop listening and go on to the next thing without wasting any more of your time on what has become bunch of fluffy nonsense? With email the recipient can (and probably will) just go on to the next message.
Try to limit your post to 1-2 screens of text. Failing that, make sure you tell only as much as you feel you must to get your point across, nothing more. Stay on topic, then end. Remember, you don't have a captive audience.
Line Length
A nice little thing to remember is that not all computers handle messages in the same way. Messages that may look perfect on your screen may well arrive completely jumbled on someone else's. You can avoid this by limiting the length of your lines to no more than 65 characters. If your email program doesn't automatically wrap the text, no problem; just press 'return' after about every 10 words to make sure your lines are short enough.
Paragraphs
Try to skip lines between paragraphs and limit yourself to 5-10 lines per paragraph. The extra space is easier on your readers' eyes, and your text will have more impact.
Quoting
It's sometimes necessary to quote a section of text from a previous message the person on the other end sent you, as a reference for your response. Be sure to quote accurately, or you may hear complaints. Quote only as much text as necessary to make your point.
Tone
Perhaps the single most important aspect of good email writing. Make sure the tone fits the woman you're writing to. Remember, there really is a living person on the other end. Remembering this simple fact will help you accomplish what you wish with whomever your recipient happens to be. Think about your readers and tell them only what they need to know in order to understand you. If you're worried about style, write principally about those things that truly interest you. Keep everything short and crisp. Above all, write simply and honestly; if it wouldn't sound right coming out of your mouth, you probably shouldn't send it.