To add on to Scott's response, and maybe bring this back down to a more basic level, I would recommend using common cents. For example, if your around a metro area listen to one of the approach/departure freqs (preferably one that corresponds to the direction you are from the main area of traffic such as ac arriving/departing from the north, south, etc.) If your in a rural area then you have several options, I usually listen to either a center freq, or a local unicom freq. I live in eastern Nebraska so I prefer to listen to an approach freq as much as I can. On the weekends I fly for a local skydiving club. The biggest concern I have is someone "cruising" thru the airspace above the airport of our operations. This normally would be of no concern for a pilot to do provided he were above the pattern alt by a 1000 feet, however it never ceases to amaze me how many pilots obviously have not checked notoms to see that there is parachute activity in the area, so they come cruising right thru the drop zone. This can be helped if that pilot is listening to approach because they are monitering the area, and letting pilots know about the activity. They also give out traffic reports to area pilots based on how busy they are. (They will try and raise you by calling in the blind if your not in contact with them) Even if they are busy they still will warn ac of possible incursions. Best advice, keep your head on a swivel, and don't daydream.

Common misconception. 123.45 is not an official air-air frequency, though you will often hear people use it as such. 122.75 and 122.85 are the official air-air frequencies within the United States. I recall seeing a use for 123.45 somewhere, but I can't find an official reference to it anywhere. I think it has something to do with oceanic flights, but I could be wrong. For the official breakdown on non-atc frequencies see AIM 4-1-11