According to AlbionPleiad.com, there is no single best way to ask a girl out, but there are a number of strategies to use to increase success. For instance, avoid pickup lines, and instead, ask about her interests.
Natural conversation in person is an effective segue into a date, especially if there are interests in common. Ask a girl for her number and for permission to text her, and take it in stride if she declines. Most girls do prefer that you ask her out in person, so work it into the conversation. Do not ask for her phone number just to ask her out by text an hour later.