It took ten days--no, eleven--for our heater to get fixed. The circuit board had crapped out, and apparently the replacement part took its sweet time over here. When our repairman finally DID come back, he took one look at our furnace filter and gave us a scolding that we probably aren't going to forget for a long, long time. Did we know that we ought to have cleaned our filters once a month? Um, really? We thought it was a seasonal thing (not that we ever remembered to do it then either)... Since that scolding, we've learned that a clean filter will keep our furnace happy and efficient--and even cut some of our energy bills. And after hosing off all the layers of caked dirt and dust on our filter, we are pleased to report that our heater is blowing 100% better than before it broke down.
For instructions on how to clean a furnace filter, check out this tutorial on eHow.