How can I show a cashier that I am interested in him?

I agree you should go in around closing time or when its not busy. Then when he's checking you out at the register say hey what's your name in a cheery voice :) Then tell him your name and tell him you've noticed it's not that busy. Then just start talking about something and get a conversation started. Then keep talking to him every time you go to that store. And if things keep going well give him ur number and tell him you guys should talk sometime. Hope I helped :)