How do you ask friends to pay you back without sounding rude?

How do you ask friends to pay you back without sounding rude?

Whenever I'm out with my friends for lunch etc and they pay the full amount, I would insist on re-paying my part to them asap. I don't like having any dues. However when I cover the bill by myself and the other person forgets to pay their part, I have a hard time asking them to pay me back. I often wait till they pay by themselves or sometimes even give up on expecting them to pay. Do you think it's fine to straight up ask someone about the money they owe you without sounding rude? Thoughts?

How do you ask friends to pay you back without sounding rude?
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