1 Samuel 16:7
"For the Lord sees not as man sees: man looks on the outward appearance, but the Lord looks on the heart."
What does this verse have to do with dressing modestly? Well modesty is about both our heart and our outward appearance. The verse above states two facts "man looks on the outward appearance" and "the Lord looks on the heart." So God is the most important, so I will start there. God sees what is in our heart, he knows if we are modest in spirit and bearing.
Now man, people, we look at the outward appearance, and by that, judge (form a first opinion/impression of) others. We are exhorted in Matthew 5:16 to; "let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven." If we look exactly as the world does than how can we be lights to others, especially those people who we walk past on the street. We are to be lights at all times and by dressing modestly we give ourselves so many more opportunities to let our light shine. The people who see us will be able to see by how we dress that we are different, we don't even have to talk to them to be a witness for Christ.
Now before I go any further I will define modesty a little, the word modesty. Modesty of a woman is; dressing or behaving so as to avoid impropriety or indecency, especially to avoid attracting sexual attention. To be and dress in a humble, satisfactory or adequate way. To wear clothing that is not revealing or emphasizing a person's figure; to be decent, seemly, demure, sober.
The Latin word that modest comes from means; "keeping due measure" or keeping within reasonable or proper limits.
On the note of dressing modestly so as not to be a distraction for christian guys, you should also be doing it for your own purity too. A lot of guys who are not christian would not see any reason that lusting after you would be bad. Do you really want to be an object of lust in some random guys eyes?
"for out of the abundance of the heart his [her] mouth speaks."
Out of the heart the mouth speaks, so out of the heart the body dresses. It is a heart issue
Even if you were modestly dressed and you heart was in the wrong place you would not be truly modest, which is why rules and legalism does not help. But if you say you have a modest heart and don't dress accordingly are you really being modest? Is your heart truly modest? Or do you just say it because every christian girl knows she should be a good girl and be modest and the easiest way is to say she is? God knows exactly what is going on in your heart. Faith without works is dead [James 2:14-26]. Having your heart in the right place but not showing it in an outward manner just does not work.
Works do not save, but we can tell if we are saved by our works, meaning the fruit that comes from our being saved, is works. It is similar with modesty as with out faith. Dressing modestly does not make us modest but by that (our dress) we can often see where a girl's heart is.
You can't have just the modest dress or the modest heart, you must have both the heart and the dress. Shine Christ's light to the world, you are set apart for God, act upon it!