Avoid fighting against a Riven in skirmishes, as her raw damage and CC will usually outpower most champions.
Riven only deals Attack Damage – so try building a lot of armor to counter this if she becomes fed.
Although Riven’s skills give her a lot of mobility, using a snare/stun during her Broken Wings or Valor will prevent her from getting away/chasing.
Use champions with long ranged harass to zone her out. Be careful not to trade while harassing her, as she will tear you apart if she has ult or you’re a level behind.