If you have a floor that has started to sag or feels "springy," so that heavy steps on one side of a room can be clearly felt through the floor on the other side of the room, the problem is not your flooring, it's the floor joists underneath. This won't occur in houses built on a slab because the joists rest on concrete. But if your ground floor is built over a basement (for the purposes of this article, we are only talking about a ground-level floor built over a basement), check out this article on how to http://www.selfhelpandmore.com/building/bracing-upstairs-floors.php">brace upstairs floors and floors built over a crawl space. Sometimes, joists develop a little "give" when there is weight overhead, and when a joist--usually a 2x8 or 2x10--moves in one place, it moves all along its length. This is what causes glasses in cabinets on the other side of the room to rattle as you walk through. In other cases, the joists start to sag in the middle and the flooring up top likewise begins to sag.
The best fix for loose or sagging joists is to brace them by installing a beam perpendicular to the joists at mid-span, which only works if your house has a basement. While it can be a little intrusive if you have a finished basement, it eliminates any flex your joists have developed.
Use boards that are the same size as your joists (2x8, 2x10) to make your beam. You need two boards that span from wall to wall across the joists. Attach the two boards together with construction adhesive and ten-penny nails. Use 2x4s to brace the beam into place. If your floor isn't sagging and you are only adding the beam for stability, install permanent posts made of pressure treated 6x6's every 10 feet (you can attach metal post bases to your basement's concrete floor with expansion bolts that make this easier). If the floor is sagging, wedge a 2x6 at a slight angle between the basement floor and the beam. Use a mallet on the base of the 2x6 to force it to straighten, pushing up on the beam to take the sag out of the floor. When the floor upstairs is as level as you can make it, install 6x6 support posts every 8 feet. Don't remove the bracing pieces until all of the posts are installed.