You don't. A man has to (want) to ask a woman out.
A number of factors come into play from his point of view. 1) Is he already involved in a relationship? 2) Is he physically attracted to you? 3) Does he believe/assume you are in a relationship? 4) Have you flirted with him or given him any indication that (you) have an interest in him? 5) Is he in a position financially to pay for the kind of date that would lead to a second date?
The first two are probably the most important ones. These are things you may never know. As for number 2 it has been stated many times that "beauty is in the eye of the beholder". Generally speaking a man will not ask a girl out who is not attracted to. You can't do anything about the first two. After physical attributes comes personality and sense of humor. (aka) Chemistry. There is no one size fits all when it comes to the traits men are looking for. Sometimes you have to make the first move. Subtle flirting with a guy is a good way to gage if there is any interest on his part. If you don't seem approachable the majority of guys will not approach you. The best advice is to (be yourself) and let the chips fall where they may. Ultimately everyone wants to be with someone who will love and appreciate them for who they are.