Push one of the levers b6 or e5, or both.
Queen's gambit. White: c4-c5
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Because it releases tension in the center and Black often has an effective ...e5 break taking over there.
Actually, c4-c5 can and has been played in the Queen's Gambit Declined in top level chess, but it is preceded by Bf4, e3, and Nf3, getting the e5 square under control and a bit further developed before locking the pawns. Then White is playing for Qside space - ideally meeting ..b6 with b4.
I have always heard that white should not do this, but when I face it as black it is deadly. How do I exploit this move as black?