A good friend of mine is a tech at a Firestone. He's told me that even when they do tire changes, they are supposed to throw the car on the alignment rack, but that is at no charge to the customer unless they request it. if you know anyone that works at shops in your area, ask them if they do that at their shop then find an excuse to get your car in for something simple so you can get a quick check on your alignment for free! of course, if the alignment is out, they have to tell the customer. My buddy likes the alignment rack deal at his shop, because the customer doesn't get charged for it, he leaves the car on the rack for about 20 minutes while doing other things, and he gets paid for an hour of labor @ his $18/hr rate.
Another thing you could try is see if you can find shops with a "lifetime alignment" package. His shop charges about $100-110 for it, but any time you think your car is out of alignment, just take it in and they'll check it for free. Hey, even if you only ever take the car in 3 times on the lifetime alignment deal, it's worth the $100-110 initial cost, as the alignment rack is an hour labor and just about every shop out there charges $45+/hr labor!