While stealthed, pay attention to when you are (near) being detected by enemy champions. This is identified by the glowing yellow and red eyes over nearby enemy champions.
Try to use Dark Frenzy after your opponents have used up their slows.
Evelynn can be built focusing on either Attack Damage which provides more sustained damage or Ability Power which provides more burst damage. A balance of both may be ideal.
Purchasing Vision Wards can help you detect Evelynn's location in order to prepare for her ambushes.
Evelynn is vulnerable to slows once she uses her Movement Speed boost, Dark Frenzy.
If you suspect Evelynn is about to ambush one of your teammates, let them know by pinging the minimap and typing in chat.