Howley will tell his unchanged starting XV that message ahead of the third and final Test at the Allianz Stadium in Sydney on Saturday (kick-off 3.05pm local, 6.05am BST).
Wales led for 71 of the 80 minutes in Melbourne last weekend, yet still lost the game when Mike Harris kicked a penalty after the hooter to make it 25-23.
But the Wallabies enjoyed almost 70% possession and the pressure told on Wales in the end.
"When you play against southern hemisphere sides, and Australia in particular, possession statistics are vital. If you are defending for 70% of the game it takes its toll," said Howley.
"We need to remain composed and accurate at line-out time because then the possession stats change. We lost four line-outs in Melbourne and we need to be more clinical in that area."
The Wallabies are aware of the need to make better use of their possession, though number eight Wycliff Palu praised the intensity of the Welsh defence last time out.
"They chopped us a lot quicker than in the first Test, especially in the forwards, and that certainly slowed up our ball. They got in there pretty quick and so we've been working on a few things at the breakdown.
"There were times when it looked like there was a red wall in front of us and their line speed was very fast."
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