So there's the content half, making sure I have what I want in the message, such as things to talk about, nice things about the other person, things about myself, etc. The other big one is HOW I'm saying it. I think how you say things online says a lot about you. Being easy to follow and understand is very different than TYPING IN ALL CAPS AND TXT SPK 4 LOOOOONG PARAGRAPHS. That's the reason I read it back and try to get a feel of it. I'll read posts here, for example, and get an idea of person based on how they type, how they present their ideas. Some are a lot more logical and conscience, with others being more fun and crazy. And some being very Scottish *cough* Graeme1988 *cough* Doing this can help with your worry about coming off as needy. Do you read it and feel "wow, this person desperately wants me to reply." If it's how you actually feel, odds are it is how it comes off, to be honest. Two ways to get around that could be either not being as needy (which can be hard) or to imagine yourself as someone who isn't, and type out your message as that person. Try to be a version of yourself you like, on present yourself as such. Definitely be yourself still, just make an effort to downplay the qualities you don't want to give off and up-play the ones you do.
Now I feel I'm starting to rant, but that's where I start when sending messages, or typing out posts. I read over this one a couple times, and hope it was helpful. I know I could have been more complete but didn't want to go on and on.