What are the best ways to clean a hardwood floor?

Lightly sand the residue.

Varnishes seep into the wood so you wont take the colour off right away like with painted surfaces. Don't sand it too heavily.

Of course if you wish to sand and buff the entire surface of a poor condition floor then a heavy sanding may be what you want however I am going to assume the residue you speak of is a result of surfacing the floor in the first place.