Whether you're sending on-site emails or SMS messages a few simple rules go a long way.
First, is your profile complete? 99.99% of the time female members do not respond to male members who have not added a picture (with their face in it) and filled out their profile info.
Next, you need to have realistic expectations about how online dating works.
Male members send about 100 times more messages to female members than the other way around! Male members sending WELL-WRITTEN messages generally get one reply to every 20 to 50 messages they send!
Finally, is your message WELL-WRITTEN? Women get copy/paste emails all day long. Take the time to personalize your message to the person you're sending it to. The basic information can be the same but put something in there to show her you actually took the time to read her profile. Women like that!
One final note on SMS: obviously it is more immediate and tougher to write a long message. But always start off with introducing yourself! "Hey, this is <yourusername> from <site>! I saw your profile..." What do you think a female would think if she got a text from some guy she doesn't know with "Nice Boobs"?