Match texture
Choose source skin that genuinely resembles the area you're repairing. Texture changes across the face, so proximity alone doesn't automatically make something a good source.
Match light and tone
The source patch needs to live in similar lighting. Sampling from a brighter or darker area can leave a tonal interruption even when the texture itself looks convincing.
Keep the blend controlled
Feather enough to make the edge disappear, but don't turn a small targeted correction into a large soft area.
Inspect movement
Scrub the shot, play it at normal speed and look closely at turns of the head, changes in expression and shifts in lighting.
Know when to stop
This is probably the most important part. Beauty retouching doesn't improve simply because you keep doing more of it. Once the distraction has disappeared and the skin still feels natural, the correction has done its job.