I usually check their profile to see if they're single. Sometimes I'll just add them with no introduction. If they add me, over the next week I might like a photo or two of theirs, comment on a few status updates or alike and see how they respond. If they like my pics or seem receptive to my comments, I'll drop them a mail, along the lines of:
"Hey there. We've not had chance to speak yet so I thought I'd drop you a mail to say hi.
So how are you and how's your weekend been?"
We swap a few mails of basic chit chat, getting to know a bit about each other etc, swap numbers and escalate from there.
A lot of the time, I'll get a response or two to my mail, but then won't hear anything again. Other times they seem really keen to get to know me. Sometimes they're even quite obsessive and seem to think I'm this perfect guy.
I've tried being ****y and funny or forward, but it rarely works. I find taking it slowly and building rapport works much better for me. A lot of guys comment on their pics with "you're fit" and "I would bang u" etc so I set myself apart this way. I think it's also important they feel comfortable with you.
I confess, I do a lot of cutting and pasting of emails to save time.