Taut pitch and slope are everything.

Make sure your rain fly is pitched taut and with a steep enough slope so water runs off, not pools.

The most common mistake I see is people leaving slack in the fly. Water pools in low spots, then the fabric sags further under the weight, and you wake up with a leak. A taut fly sheds water. Period.

Use trekking poles or a dedicated pole system to create a high ridgeline. Guy out all corners and side pull-outs so the fly is stretched drum-tight. If possible, pitch the fly so it doesn’t touch the inner tent—contact points wick moisture through. Also angle the fly so one side is higher than the other; water follows gravity. Even a slight slope beats a flat roof.

A saggy fly is a soggy tent.