What could be some early signs that the person may be getting into an abusive relationship? I feel it is important to keep them in our radar and avoid the pain and heartbreak. While no sweeping generalisations can be made, perhaps certains traits or behaviours should send the alarm bells ringing, and make one more cautious.
I'd say any extreme behaviours, or sudden ticks. Like extreme possessiveness or insecurity. Also, see how they behave with other close ones in their lives. If you become a close on, they'll treat you the same way*p*