Ponte guapo

Ponte guapo literally means "make yourself handsome". Perhaps a more proper interpretation is "make yourself look good". It is used as an invitation for somebody to do something nice (for a change) and treat the person using the expression. 

A related, more formal expression is cortar una rosa de tu jardín. It also means to treat somebody. 

Example:

Lionel: Póngase guapo y saque las caguamas.
Xavier: Sí, profe, que se corte una rosa de su jardín.
Josep: Sale. Si ganamos prometo que yo pongo las frías.

Lionel: Do something nice and buy us a few rounds of 40-ouncers. 

Xavier: C'mon, coach, treat us this one time.

Josep: OK. If we win I will buy the booze.