Annie Thornton First make sure that this is not a structural issue, and that the floor is not sagging. To fix this problem there are a few things that you could do.
-First, you could remove the baseboards and remount them lower to fill the gap.
-You can also install shoe moulding to the bottom of the existing baseboards, and, depending upon the gap, this will cover it.
-Some people do not like the appearance of shoe moulding so a third option would be to fill the gaps with thin pieces of wood with adhesive on the top. Caulk the gaps after the adhesive dries, and when the caulk has tried you can paint a smooth finish. For door moulding and columns, consider plinths.
http://www.sherwin-williams.com/do_it_yourself/paint_colors/ideas/color/SW6252_ice_cube/