Remind them once – if you have to remind them more than once they’re a piece of shit and not a friend. In fact, if they’re the type of person who doesn’t pay or always chisels you, make a point of hanging with them socially in a situation where they put their card down for the outing and then just don’t offer to pay them back – they won’t ask you for anything in return unless they’re absolutely shameless because they know they owe you.