What to wear
For the owner: plain and in a colour that contrasts with the animal's coat. Black clothing with a black dog produces one indistinct shape. Also…
In short: For the owner: plain and in a colour that contrasts with the animal's coat. Black clothing with a black dog produces one indistinct shape. Also expect fur on everything, so avoid anything that shows it.
Colour. For the owner, whatever contrasts with the animal's coat. Black clothing with a black dog produces one shape with no boundary between them.
Fabric. Something that does not show fur, because it will. Smooth cotton over wool or fleece.
Footwear. Walkable and washable. Good pet frames happen outdoors and at ground level, which means kneeling in whatever is there.
The thing that ruins frames. A bright lead or a plastic harness in a portrait. Swap for a plain leather or rope lead, or drop it out of frame entirely if it is safe.
In the bag. High-value treats the animal does not get at home, a squeaky toy kept out of sight until the exact moment you need ears up, and a lint roller. A second person is worth more than any of it.