Cross my heart used to emphasize the truthfulness and sincerity of what one is saying.
cross one's fingers put one finger across another as a sign of hoping for good luck.
cross someone's palm with silver pay someone before having one's fortune told, originally by describing a cross on the fortune-teller's hand with a silver coin.
cross swords have an argument or dispute.
cross the floor in the British House of Commons, to change one's party allegiance, literally by moving across the floor or open space which divides the Government and the Opposition benches.
don't cross the bridge till you come to it do not concern yourself with difficulties until they arise. The saying is recorded from the mid 19th century, but is now also common as the metaphorical phrase to cross one's bridges when one comes to them.