So why do I want to check the bearings?, you are thinking. (See the diagram below) The bearing is the roller that is inside the wheel hub. The wheel actually rolls around the bearing. The bearing needs to be snug, but NOT too tight. A loose bearing will cause additional "play" within the hub. This means the wheel’s rotational axis can shift around a bit and this throws off the steering and handling geometry. This will also cause additional wear to the bearing. Conversely, a bearing that is too tight could potentially cause enough extra friction to limit the wheel’s rolling friction. This could lead to increased bearing temperatures and bearing failure.

Tools Required to check the bearings:A jack and your own hands.

Additional tools required to make the adjustment (IF NECESSARY) = wheel nut wrench (see the "What’s in your trunk [boot] section"—J.K.), needle-nose pliers, flat screwdriver, and a Torque wrench (optional).

Check Procedure:

Jack up the car so one of the rear wheels just rises off the ground.
Be sure the Parking Brake is NOT engaged! Turn the wheel a bit to confirm this.
Check the wheel bearing’s "play."
To do this, grab the tire with both hands, one on each side of it (top & bottom or forward & rear sides). If it is solid (without play) then you are done and your bearing is adjusted fine. If you can shake the wheel and feel play in the bearing, you should then proceed with "adjustment."

Adjustment Procedure:

Remove the wheel—put a paper towel between the tire iron and the lug nut to avoid scratching it up. Avoiding scratches will help keep rust from forming on the scratched areas.
Set the wheel aside— to avoid scratches be careful NOT to set the outward painted surface of the wheel on the ground!
Marvel at the up-close viewing of one of the world’s most incredible suspension set-ups!
Check your brake pad and rotor condition while you are here.
Use the flat-end screwdriver to pry out the dust cap from the hub center. Coax the caps out gently by putting the blade of the screwdriver between the hub and the cap--slowly turn the wheel around as you pry, to make it come out evenly--it goes a bit deeper than you'd expect.
Using the pliers, remove the securing split pin from the nut. Carefully straighten the pin; then put a long shafted thin screwdriver through the eye; rotate it gently whilst tapping the shaft with a blunt object. Work it out slowly--patience will win over cut fingers and scratched hubs!
With a socket torque wrench, tighten the hub nut to 18 lb.-ft. (25 Nm) WHILE rotating the wheel—this is the step that snugs up the bearing!
Loosen the nut and then re-tighten using only fingers to "finger-tight."
Insert the securing split pin. If needed, turn the nut the smallest amount further to allow re-insertion of the securing pin through the nut. You'll notice that the nut is notched and that there are a couple of cross drills through the center pin. I found that moving it around one notch to the next drilled hole for the split pin was enough--this is the least you can move it! Gently rotate the pin back and forward as you insert it and it will pass more easily. Bend the short end over the nut and the long end over the end of the axle.
Refit the hub dust cap and replace the wheel.
Check the bearing’s play again to be sure you did the right thing and feel the difference.
Because you are tightening the bearing nut and then loosening it to a final position of only "finger tight", you can see why a torque wrench is not absolutely necessary—just be careful! My bearings were fine, but Steve P. said he needed to adjust his. He said it made a noticeable difference in the play of the bearing and handling of the car.

Charlie has the following to add:

"It took me about 30 minutes to do both wheels. It does make a difference--the steering is tighter. I've been getting some excessive steering wheel vibration which I suspect is the brakes, perhaps a rotor, since I can feel it on the pedal and more when I brake; but there's definitely less of it at lower speeds. I guess the thing about problems like loose wheel bearings is that it takes a long time to happen, so you probably don't notice the gradual drop in performance of your car--but you'll notice it when you've taken up the slack!

"Drive the car around the block a few times after you've adjusted the bearings and check the wheels for heat. They ought to be slightly warm (that is if you live in California; God knows how long it takes for the back wheels to warm up in Ossett, Yorkshire!). If they are more than lukewarm, you've overtightened the nut, so loosen it up again. I drove about 20 minutes on the freeway at about 90mph and they were warmish; unlike the fronts, of course, upon which you could happily fry eggs. I'd take the temperatures with my two digital thermometers, but someone washed them in the sink, with the rest of the dishes--I'm not saying who for fear of reprisals."