Why You Should Praise Your Ex-Girlfriend
Never speak negatively about previous girlfriends or dates or your ex-wife, if you're divorced.
The girl you are with will just assume that you'll be doing the same thing about her behind her back. In fact, trying never to say anything bad about anyone in her presence is a strategy that will never do you in.
However, please don't go on and on all "poetical-like" about your last girlfriend and how wonderful she was. If she gets the impression that you're still in love with an ex, she'll lose interest in you immediately.
Complimenting Girls the Right Way
Many guys inadvertently insult women while "trying" to compliment them. For instance, "You look beautiful today!" While meaning well, you imply that she does not usually look all that beautiful.
(I've seen this particular disaster before. And the girl was not happy.)
A better compliment would be, "Lisa, you look even more stunning today than you usually do!"
Can you feel the difference?
She certainly will!