Make sure you personalise the message. Make it a decent length, but not too long as everyone's concentration range don't stretch past 20 seconds or two paragraphs.
Start with something like.
'Hey, your profile caught my attention and I thought I'd drop a message and say hello.'
Then proceed to write a line or two about their hobbies and a line or two about yourself.
End with something like.
'Anyway, hope to hear from you. If not, happy hunting' Or something like that. Light-hearted, non-committal.
Make sure your spelling is near enough spot on and use punctuation.
Lastly, make sure you have your best picture up there. No picture, no reply, I don't give a **** if you write like Shakespeare. Show your mug. No sunglass poses either as it don't give an accurate representation.