1. Put it in "D". Drive away.
2. If there's a long stop, put it in "N". A friend bump a car cause he fell asleep and he forgot he left it in "D".
3. Check weekly your AT fluid level.

The 1 and 2 are for uphill driving. If the car is having a hard time, put it in 2, it will remain in 2. Kung kulang pa, move to 1.

Regarding shifting often from D to N or vice-versa and causing damage in the end, my opinion is: that's what gears for! Automatic trannies were invented so a driver is free from a clutch pedal.

Just maintain a good level of AT fluid and you'll be OK regardless of shifting most of the time or not.