Know going value of items.
Buy for slightly lower.
Sell for slightly higher, but still making a slight profit.

For example;
If an item sells for 5k
You want to try to get it for 4k
Push it back out at 4.5-5k

Always have a set target range. I only ever had a few items not go out on an personal auction. Mainly high-end items such as Green Clover Pin, PL Sash/Boas/Pins. Everything else, as long as it has a decent outgoing rate, should generally sell same/next day.