If the top is sheer, open or not there: start with pasties. It is the piece the rest of the outfit depends on, and the one people leave to the last minute and then cannot find in the finish they wanted.
If it sticks to skin: clean, dry, no lotion or oil. Adhesive is what fails on a hot day, not the product. Reusable silicone backings hold longer than single-use paper, and spares in a bag cost you nothing.
If you want the outfit to look different without buying another outfit: jewels, glitter and leg wear. They change how a set reads without replacing any part of it, which is why they get rebought more often than anything else in the category.
If the heat is the problem: fishnets and mesh over anything solid. You get a layer on the leg or the arm and you give up no airflow at all.
If a plain set needs structure: a harness over the top. Straps change a silhouette faster than another garment does, and they go over what you already own.
If you need your hands free: wearable bags only. Anything carried gets put down, and anything put down in a crowd is gone. See festival tops and festival bottoms if the set itself still needs building.