1. Save the Biotic Grenade for 2 things (Cancelling enemy healers, and healing yourself)
2. Aim the Sleep Dart at where they are GOING, not where they ARE.
3. Nano Boost as fast as possible, but do not waste it.
4. Your teammates are not as aware that they have a healer as they would be because there is nothing to tell them a healer is around (No Lucio circle, Mercy health bar, Zen orb, etc) so heal them as they fight, as opposed to when they need it or else they will leave the fight and look for health packs.
5. STAY AS FAR AS POSSIBLE (seriously, Ana does the same damage no matter how far, and she has NO way to escape.
Follow this kind of stuff, look up some pro Ana plays from the Overwatch League and thank you for playing healer! <3