A non drying oil based clay should be treated with release before making a polyurethane mold. Although the polyurethane will release from the clay without release, the clay will stick in spots and you will have to spend more time cleaning out the rubber mold. Use Pol-Ease® 2300 Release Agent and brush it into all the details carefully so you don't damage the clay. Don't use Pol-Ease® 2350 Sealer & Release Agent or PolyCoat Sealer & Release Agent as these may soften the surface of the clay and damage your model.
If you used a water-based clay that has dried out, you will need to seal the model with Poly PVA Solution first, then apply Pol-Ease 2300. If the clay is still a little moist, a heavy coat of Pol-Ease 2300 brushed around well should be fine. The risk is that moisture will cause the polyurethane rubber to foam or bubble and give you a bubbly surface on your mold. Water-based clay is not recommended for polyurethane rubber mold making.
Silicone rubbers can be poured or brushed against an oil-based or water-based clay with no additional preparation, as these are not moisture sensitive. However, if you are using a Platinum silicone the clay must be sulfur-free, and you should always do a small test cure first to be sure there won't be any inhibition. If the clay is very dry you may want to apply a coat of PVA to prevent the silicone from sticking to the dry surface.