Subsurface extension pipes are an important first step, but if the floor is visibly moist--chronically--then installing a system of foundation drainage pipes connected to a sump pump is a cost effective way to keep the basement dry. Waterproofing the exterior wall below grade and installing foundation drainage pipes can be very effective, but is also a more expensive solution. If you install a sump pump, consider a back-up generator in case you lose electricity during storms.