So you might've had a heart attack over the weekend when you were driving by the gas station and realized that prices popped up by $0.15 to $0.20 or so. Sure, $3.30 in Little Rhody, but you can make yourself feel better by remembering that Europeans are paying around $8 a gallon...
Luckily, as gas prices increased 5% from one day to the next, you can decrease your gas consumption by a similar number without much work.
Your tires. They're filled with air, and they need to be. Underinflated tires give you worse handling, significantly more wear and tear on the tires causing early and pricey replacement, and of course steal easily obtainable mpgs from you. If you haven't checked them recently, you're likely below the recommended tire pressure since a. admit it, you may not have felt like checking ("I don't even know how!"), and b. it's winter, so your psi will naturally decrease with the colder temperatures.
To find the pressure in psi (pounds per square inch) to which your tires should be inflated, check the chart likely stuck on the inside of your driver's side door. If not there, check the owner's manual.
Rather than give a step by step, here's a convenient write up at eHow.com. If you don't have a tire gauge, you can buy one for $5 at your local hardware store (oh, go support the little guy!). Gas stations are starting to charge for air, so try to find one that doesn't.
Also, it is possible to inflate your tires slightly beyond the recommended pressure. Firstly, realize this is at your own risk. Secondly, the previous statement is largely because I imagine I have to say that. Thirdly, realize that is a safe practice that can improve handling and increase mpgs by a small but noteworthy percentage. You will feel an ever-so-slightly harsher ride, but I can assure you it's nothing anyone would even notice unless you pointed it out.
On my Prius, the factory recommendation is 35 psi front and 33 rear (there's more weight in the front of the Prius), and I have mine at 41 front and 39 rear. This is standard for many Prius drivers, and is not a personal experiment by any means. The max sidewall listed is 44 psi on this particular tire. It is important to never exceed this number (even though they are designed and produced to withstand it), since any gains beyond a slight extra inflation provide little boost to your mileage, and there is the extreme danger (not just inconvenience, mind you, but real danger) that could arise if your tire ever blew out.