Tires naturally lose 1 PSI (pound per square inch) of pressure each month.  Add to this temperature changes and the trend is even worse; for every ten degrees of temperature drop, the tires lose another 1 PSI.  This means that in only 5-months, tires could lose roughly 10 PSI, or approximately 30% of their recommended pressure.

When tires are underinflated they heat up more than they should, which can lead to premature aging of the tire, abnormal wear and even potentially catastrophic blow-outs.

Tire pressure should be checked at least once per month (including the spare tire) and more frequently if the temperature drops.  For proper measurement, tire pressure should be checked before driving as the tire is at a cooler state before driving; tire pressure will read higher on warmed-up tires.  Statistics say that 3 of every 4 people wash their car at least once per month, while only 1 in 7 check their tire pressure. 

Tires should be inflated based on the manufacturers specifications and not simply based on what the tire says - tires show a general point for inflation and a maximum point that should never be exceeded.  For most vehicles, the correct tire inflation information can be found inside the drivers side door frame; for others, this information is inside the little door that covers the gas cap.  When looking at this information, take note if the front tires require more or less air than the rear tires - this is very important.  Additionally, some vehicles show different pressures if you're carrying a lot of cargo.  Underinflated tires in a vehicle carrying a lot of passengers or cargo is very dangerous.

For ideal pressure assessment, invest in a digital tire pressure gauge (around $10 at Radio Shack) - they typically will measure 1/2-pound increments and are preferable to the cheaper mechanical kind that tire stores give away.

In addition to tire pressure, tires should be rotated every 5,000-8,000 miles; it's usually easiest to associate tire rotation with oil changes.  Many shops who sell tires offer free tire rotation and tire balancing for the life of the tires - I'm sure you could also have them check your tire pressure regularly too if you're uncertain how to do it.

Since we're talking so much about tires, let's also not forget when and how to check for the proper time to replace them.  Visual inspection is the first step - if you can see bars of continuous rubber that are at the level of the tread, it's definitely time to replace them.  Abnormal patterns of wear may indicate overinflation, underinflation, alignment problems, etc.  If the tire is old and is showing small cracks around the sidewall of the tire, this too means that it's time to replace them.  Tires not only have limitations of wear, they also have a shelf-life.  Ozone damages the tire rubber and results in premature aging.

In purchasing tires, I find it best to shop online and determine the best price available.  Then, find a local shop that you trust and find out their price.  If their price is higher, let them know that you found them for less online and were curious if they would match the online price - I haven't found anyone yet who won't do this for you.

If you would like to learn more about tire safety, take a look at:

http://www.tiresafety.com/

http://www.nhtsa.dot.gov/cars/rules/TireSafety/ridesonit/brochure.html

and if you would like to check out the database of tire pressures for your car, go to http://www.drivegreen.com/pressureData.shtml

images from TireSafety.com