I haven't traveled out of Pittsburgh for years, so have no specific advice for that particular airport. BUT, that being said...
Have a back-up plan.
Have a back-up to the back-up.
Be prepared with info on ALL the airport lots. Know how much they would cost if you have to use more expensive parking, like a daily lot or airport valet. Don't waste time at the airport dithering about if you should spend the money on a more-expensive parking option. Time is money and you don't want to lose out on your holiday because you couldn't stomach spending $15/day on parking. Have the info with you and be prepared to act on it.
Do research NOW on any off-site ariport park'n'fly lots. See if they offer online coupons.
Do research NOW on any airport hotels that offer stay one night get free parking + shuttle to airport. Know before you start out from your house how much that possibility would cost you. See if there are any deals/coupons.
Listen to the local traffic/weather radio station the day before, throughout your travel day and as you drive to the airport. Many times around the holidays they give updates on airport & parking conditions.
Before you leave the house, check the airport's website. They might have parking lot conditions & security times posted there.
See if there is an airport AM radio station that broadcasts parking lot conditions.
Your arrival time at the airport...ummm...how do I say this....get there earlier than you think you would ever have to. Plan on spending AT LEAST one hour to get through Security. Plan on spending AT LEAST one hour checking your bag/s. Plan on driving around the parking lot for AT LEAST 20 minutes looking for a space.
If I were you, I would get to the airport (with having to park your car, unload your bags, get on a shuttle, get to the terminal, dealing with crowds, checking your bags, etc.) much earlier than 6pm. Actually, I would get there earlier than 5.
agnes!