Go to Notifications.
Go to Messages.
Turn "Show Preview" off.
Now, whenever you get a message, it'll show up like this on an unlocked screen...
...and like this on the lock screen. Note: Without the tie, of course.
If you pull down the Notification Center, the preview will still be there. So, if you are really concerned about privacy, take iMessage out of the Notification Center.
This is great if you are talking about a private subject, or have people constantly looking over your shoulders.
The creator of this guide has not included tools
In the world, but not of it.