You shall not commit adultery.
Alright this seems easy like an easy command to keep, too, just don't go sleeping with someone you're not supposed to sleep with, and you're fine, right? right? Well, again, it's not that simple. Jesus says:
"You have heard that it was said, 'You shall not commit adultery.' But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart. If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell."
Matthew 5:27-30 (ESV)
By the high standards of the Lord, to look with lust is the same as actually doing the things you fantasize about. We go a long way in this if we remove ourselves from situations which might lead us to look with lust. Sex is everywhere in our society, and is used to sell just about everything; and our media sets few limits on where the lines are as far flashy outfits, tight clothing and how much skin makes it to the silver screen. We must be vigilant in what we chose to set before our eyes.
David, the King after God's own heart, ended up lusting over a married woman. He knew his lust was wrong and repented. That'd be a great lesson, even if it ended there; if you fail, repent and turn from the sin. Unfortunately, lust proved to be slippery slope for the favorite King, and ended up putting her (Bathsheba's) husband in a situation he wouldn't survive, so that he could then marry her. Nathan the prophet had the unpleasant task of pointing out to King his folly, and delivering the message of what punishment the Lord would send down on David. When David was finally made to face his actions, he again repented and was forgiven by God's mercy, but not without punishment. the death of a beloved son and battle over the order of succession of David's crown.
If you happen to find yourself tempted or even looking with lust, repent and turn from the sin. We also learn from Joseph to even try to avoid things that could be construed as impropriety.
On the other foot, we should all endeavorer to wear modest clothing, and teach the next generation what is and isn't appropriate attire. We must draw the lines that our children don't see in TV shows, movies and their favorite musician's wardrobes. We may not be able to control what others wear out of the house, but my daughter will not be leaving this house in mini-skirt, low cut top or with something written across her butt.