It is painfully inevitable. No matter how much you try to protect your hardwood flooring from stains and damage, at some point or another, someone will spill something on floor. When this happens, there's no reason to go ballistic.
If you have been diligently checking your floor's finishing, then spilling water or any form of liquid into it should not be too much of a problem, so long as you wipe the spill up immediately. But if you're floor coating has thinned and liquid has been there for quite a while, that's when you should start to think of restoration strategies.
Hardwood floor's number one enemy is water. Your floor's luster is at its mercy. You can easily remedy this by mopping the affected area. In worse cases, water can seep into the woodwork and result in decay and warping.
When wiping, use only soft, dry cloth rags. Coarse cleaning materials will scratch the flooring's surface and thin out the finishing. If you're dealing with a puddle, start with the edges of the puddle first and work your way to the middle.
Some people use steam cleaners to deal with water spills. However, you should realize that not all steam cleaners are compatible with hardwood floors. Neglecting to check compatibility might cause more harm than good to your floor. Also, no matter the nature of a stain, never use wet cloths for clean-up. Even slightly wet mops are a no-no, especially for hardwood floors that are sealed and coated with wax.
Finally, your best defense against the hassle of dealing with water stains is to be careful. But you really can't put all your eggs into just protecting your floor from water. As mentioned earlier, liquid stains are inevitable. Thus, foresight is your friend. Ensure that your floor is waxed regularly and the finishing reapplied from time to time.