I'd mostly agree with the advice above, with a couple of exceptions. I think a follow up email isn't necessarily a bad idea. Pushiness is horrible but I've sent a second email a week or two after the first, and got a response and a date out of it. Some women just have lots of emails to go through, and/or have busy lives.
If they like the tone of the second (I thought I'd drop you a second mail because .. well, why not - I thought you looked great and still do) and it's not pushy or smacks of entitlement then at worst you're slightly annoying someone who didn't like you anyway, and at best you're showing that you're not sending generic spam, and may peak her interest.
Don't come across as angry, bitter, sad, lonely or desperate.
One thing you'll have to realise is that sending a lovingly handcrafted first email to a woman on OD sites is a waste of time. Many profiles are defunct, the woman hasn't logged in in ages. Many women haven't subscribed to read the messages. And of course some will never respond. The chances are you'll be contacting the more attractive, interesting women, and all the other blokes out there will be too. They'll have lots to read through. They may never notice yours. The fact is, as a man you will have to mail dozens of women to get a response. Women generally don't make contact first - or at least the ones you fancy don't.
So, have a partially generic mail. Writing a personalised three sentence email, customised to each of the women you're mailing, and sending several of these a night - and generally getting no response back - that's soul destroying.
Look at the profiles you like. Write a mail that starts with a generic, witty opener, then in the second paragraph say something about their profile, maybe something you have in common.
Finish with a fairly generic comment, ideally asking something about them, but it doesn't have to be personalised.
As I said, you are expected to do all of the first messaging, generally. So to save time and energy, just do the semi-generic thing. I've lost count of the times I've written a personalised email, spent an hour agonisingly handcrafting it, making it specific to the person in the profile I'm interested in, and.. nothing. Nada. Turns out she probably wasn't even subscribed and couldn't read the mail, or was possibly just not interested. Waste of time.
Sending an obviously generic mail is rude. Make it personal, make it about her. But you don't have to make it all about her. Copy and paste bits of it. It's not so romantic, but it reduces wrist-sprain.