Starch and stencil: Simple things like sheets, layettes and bits of lace around the home can be starched to give them ghost like shapes. Stencil in some features and you have the boo factor down. These things do not need much time or effort or even too much money spent on them to work.
Batty ideas work: Bats are a big part of what creates the scary effect during Halloween. All you need to do is cut up some stiff black chart paper in bat like shapes and stick them on randomly all over the place to make it look like your place is ridden with them. If you are looking to take the scare factor up a notch, just put in eyes that glow in the dark with fluorescent pencils and you have a good scare going on.