Which wheel or wheels get the chocks?
To be most effective chocks should be placed on both of the main gear of light aircraft. This prevents the aircraft from rotating or pivoting about the chocked wheel.There's a nice article from the Flight Safety Foundation which talks about this.
The FAA, in an overabundance of caution, advises you to chock ALL the wheels (see AC 20-35C).



Practically speaking, on a flat ramp and a calm day chocking any single wheel is usually enough to keep the airplane where you put it for a short while (say long enough refuel the aircraft, or to get lunch).
When using a single chock my preference in these cases is to chock the nose wheel as it's the most visible (I fly a low-wing airplane, and an inattentive line person who needs to tow the plane could miss a chock on the main gear). With two chocks the main gear should be secured on each side of the aircraft to keep it from pivoting.

For anything longer than that a brief food and fuel stop, or when high winds are anticipated, aircraft should be properly secured (by tying them down and chocking at least one wheel).
In these cases the tiedowns serve as the primary securing mechanism, and the chocks as a secondary mechanism (keeping the plane from rocking fore and aft in high winds and breaking the tiedown ropes).


  Note that there is an exception to the "high winds" bit here: If winds strong enough to blow chocks around on the ramp are anticipated, such as a hurricane, the chocks should be removed unless they can be adequately secured to ensure they don't get blown into an aircraft.
  Of course if winds that strong are anticipated you should really have flown away before they got to you, or you should be looking for a hangar to weather the storm in.