Do not talk about any of your personal (family, work, money) problems.
Don't act desperate.
Don't talk about anything negative at all. Keep everything you talk about upbeat and positive.
Don't get drunk. This really turns women off and makes a bad impression. Don't drink at all or limit your drinking to a couple of drinks.
Don't try to analyse her problems.
Don't bitch about your previous girlfriends or ex-wives on how they mistreated you, dumped you, cheated on you, took you to the cleaners on child support, etc.
Don't pretend to be funny or humorous. If you are putting on an act, it will come across to her as phoney. Just be friendly and smile (but in appropriate doses, avoid having a grin on your face ALL THE TIME, guys that are smiling for hours in a row look terribly insecure and just plain pathetic).
Don't focus on your health problems or ailments.
Don't brag about yourself and your accomplishments. There are exceptions though, but try to be careful about them. If you really have something impressive to talk about, you can let her in on it, just try to be real casual about it. On the whole, women are very good at sniffing out when the male is trying to impress them by bragging. And if what he is bragging about is not really that impressive, it's a complete turn-off. So be careful. Focusing on her and her interests is a far safer route to take.
And at all costs, avoid these three themes: politics, religion and conspiracy theories. These are guaranteed to nuke your otherwise wonderful interaction.