Always seal your paint container tightly between paint jobs. This will help prevent your paint from thickening or evaporating. To seal a can properly, clean excess paint from the rim with a brush and then gently tap the edges of the cover with a hammer.
Look for dirty spots in your insulation, which often indicate holes where air leaks into and out of your house. You can seal the holes by stapling sheets of plastic over the holes and caulking the edges of the plastic.