If it's oil based then pour a little paint thinner over the paint in the can, just a film, it will keep the paint from skinning, then put a rag or paper over the lid and step on it firmly to seal the lid. the paper or rag is to keep any paint on the seal or lid from getting on your shoes.
If is water based, try the same thing but with water. then float a burning tea-light candle on the surface and close the lid....the oxygen will be consumed by the candle and the inside of the can won't rust and the water will keep the paint from skinning