Summarize this article in one sentence.
The most simple option is being very direct. Approach the person, start a conversation, and make a comment along the lines of "you know, I've really enjoyed talking to you. Do you think I could have your phone number and we could do it again sometime?" If you are asking a stranger, don't worry about having a lengthy conversation. The person likely knows that you are approaching them out of romantic interest. If you know this person before asking for their number, make sure your intentions are clear. You don't want to come across as a platonic friend looking for support when you are really romantically interested in them. Say that you need to borrow something, for example a pen, and then ask if you can have their phone number so you can call them to meet and give whatever the item was back to them. This is simultaneously flirty and straightforward, a perfect combination. Instead of asking a yes-or-no "can I have your number?" question, offer two choices. "Could I have your phone number or email?" This way, they are more likely to give you a definite means of contacting them. Although business cards are less commonly held by today's youth, swapping business cards is a great way to get someone's contact. It also verifies that their number is real, not a fake one given because they aren't interested.
Try being direct. Ask to borrow something. Ask with a choice. Swap business cards.