Make sure your house smells sweet.
Pet smells, food smells, stinks and stenches will put people off.
Keep your house smelling nice. Keeping it clean is essential - wash out any bins, clean the bathroom, make sure dinner isn't cooking when people view.
Introduce nice smells. Freshly squeezed oranges smell amazing, bread baking is an old one - but risky if you let it burn, flowers are a nice though lilies can be operpowering.
Pot pourri is useful but not in excess,