I live in a somewhat older house, and the floors (like most houses, I think) tend to creak if you step on certain spots. Well, my wife goes to bed rather early some nights, and when I come in to the room a bit later, the creaking could wake the dead. Other than just avoiding the creaky spots, is there any “fix” for this (other than tearing up and replacing the floorboards)? And what causes the phenomenon of floor creak anyway? Is it just the age and subsequent slight warping of the wood? Humidity? How can I sneak in to the bedroom without being detected by the floor’s telltale creak?
I know the answer, but your wife has sworn me to secrecy. 
There is a kit you can buy to easily fix your floors. Basically, you drive screws through the floorboards (countersinking them slightly) and into the joists below, which stops the squeaks. On wood floors, you will probably have to use a bit of wood filler inside the holes left by the countersinking of the screws. On carpeting, you just fluff up the pile.
The kit costs about $30; you can probably find it at Home Depot or some other hardware/home center.
Do you have exposed floor joists? In other words, can you see them from the basement or crawlspace or are they covered by a downstairs ceiling?
If they are exposed, all you need to do is buy or make shims and put them between the flooring and the joists.
Another method employs vinyl coated nails called “sinkers”. The coating on these nails melts from the friction of driving and effectively glues the nail in place. The shim method is better in that you can solve the problem in a reversible fashion.
The biggest problem with sinkers is if you don’t drive them correctly, all the way, first time around. Kind of hard to pull back out, they are.