First, sweep the area of dust and debris. Having done this, give the floor a thorough visual inspection for any defects. The sort of thing you are looking for are protruding nail or screw heads. If found, these should either be hammered down or countersunk below the surface of the sub-floor.
Walk around the whole area taking note of any squeaking, creaking, movement or loose boards. If identified, these issues should be resolved by either screwing or nailing down any loose or squeaky boards and even replacing split or broken planks where necessary. Floorboards should be screwed down to the underlying joists every 15 mm (6 inches)
Crucial to a quality installation of hardwood floors is a level sub-floor. Manufacturers recommend that over a span of 3 m (10 feet) there should not be any dips or humps greater than 6 mm (1/4 inch). Small humps and raised areas can be sanded down with a belt or orbital sander. To level any dips you should use a two part latex leveling compound.
Before doing this you should check for any gaps between boards or holes that would allow the liquid compound to leak through and seal these with some kind of filler, mortar or sealant. When the two parts of the latex compound are mixed together it should have a consistency much like that of a pancake batter. This allows it to run and flow to create a smooth, level surface when set and dry.
If the wooden sub-floor is on the ground floor, the chances are that there will be concrete underneath it. This means that you will need to consider the moisture levels that this may be emitting.
Solid wood floors will absorb moisture from both the air and the sub-floor that is below causing it to expand. For this reason an expansion gap should be left around the entire perimeter of the floor including under door frames and around fire places etc. To calculate the size of the gap you will require you should allow 4 mm for every meter of the room in the direction you are working to with a minimum of 10 mm.
The easiest way to prevent moisture migrating from the sub-floor is to use a combined underlay with integrated vapour barrier. This will also provide some thermal insulation and helps to dampen sound.