Please, don't use the insulting one liners...

Have coffee there often (good suggestion)

Make eye contact when possible

You'll get a "vibe" whether or not to introduce yourself

Once you get the "I'm approachable and you can talk to me" vibe---then introduce yourself.

The young lady will get the idea that you're interested because you are showing up daily during her shift...she'll let you know if she is interested.