Greg,
Here's a short list. A lot of this is obvious, but we sometimes let things go.
Seat
It should fit you. If you are sharing a car with someone else as we often do in endurance racing, you should take the time at the shop or at the track to build an insert that makes the seat as close a fit to your body as possible. Try to eliminate pressure points. Try to build your insert so that forces would distribute evenly. Use a high-density foam. If the team doesn't have any, buy some in advance and bring it. A razor blade cutting tool and some racers tape are good for trimming the bits and assembling the puzzle. Keep in mind your insert will not be flame resistant unless it is wrapped in nomex fabric. The pro teams make inserts with bead seat material. That's a little safer, but at the very least, you should be meticulous and make your own when sharing a car.
If its your own seat, no excuses. For sports cars buy a seat that actually fits. Return it if it doesn't. Try them on for size at trade shows like PRI (December in Orlando) or maybe the SEMA show in Vegas. If you are driving a formula type car, the minimum you should aim for is a bead seat, poured by someone who has done them before. If you want to be absolutely sure its right, call a company like Bald Spot Sports (Indianapolis) or one of their main competitors (most are based in Indy) and ask them if there are local experts who can build your seat, or even better, hire one of their seat makers directly. That will cost the seat material, labor, and travel expense, which can be as much as $2500 in total. You can spend a lot less (the kit is around $350) if you find a local guy who knows what he's doing. If you're not sure, look at seats they have already built. It will be pretty obvious. Still not sure? Look at a seat inside a top Indy Lights, IndyCar, ALMS prototype if you can get close enough.
Car Interior
The inside of your car can have many impact points you have not given thought to. Inspect the footwell all the way up to the steering. Formula cars may have bulkheads that are not covered in enough high density padding. Flat planes are the best. If you own the car, install carbon or aluminum sheets (you'll probably have to custom cut), and cover those in high density foam. If you have a left or right side impact, your legs will not stop until they hit something. You want that impact to be cushioned and evenly spread.
Think also about where your hands and arms might fly in an impact. Any hard or sharp surfaces? Dashboard? Roll cage? You can't cover them all, but make what you can safer. Adding a drinks bottle? Is it secure so that it doesn't become a bullet in a big hit? Again, flat planes and padding are good. In our LMP2 car we added a metal panel with padding to protect our left hands from hitting the dash switchboard, which had a sharp carbon edge right next to our left fingers.
Car Exterior
No substitute for good preparation. All humans make mistakes, but the really good teams make them far less often. Remember the big fire I had in 2009? Fuel cap was not installed. Some mechanical failures are a series of missed checks that finally lead to something breaking. Very rare, and even more rare with good, meticulous, prep. The night checks you see the Skippy mechanics doing are just this kind of preventative maintenance. Simply nutting and bolting the car every day will almost always find problems before they happen.
Seat Belts
You should be extra meticulous when belting up. The lap belt should be extra tight before you tighten the shoulders. The shoulder belts should not fall off your HANS. If they do, its possible you are not tightening the belts enough (especially the lap belt). If its your car, you can look at the way the belts are installed. Visit the HANS website for more advice.
Helmet, Suit, arm restraints
You guys know plenty about helmets. Just make sure the fit is perfect. Fire doesn't care if its a hot day, so wear your nomex underwear. Be sure headsocks are actually tucked into the collar of your suit, especially after you slip on the HANS, which can push the collar down. Holes in your gloves channel heat in a big fire. If you are concerned about fire, if you get holes, get new gloves. Arm restraints are a pain at first, but super easy to use after time. Try to buy something not too bulky. The two-piece version is a little easier to escape the car in a big fire.
Car Walk-Around
Jim Pace taught me years ago to always do a quick car walk-around each time you get in. Look for anything missing or out of alignment. Once you're around the cars enough, you'll know. If you're not sure about something, its perfectly fine to ask someone. Just might save you from losing a little track time, and more importantly, from getting hurt.
Pat, how's my word count?!
Hope that helps!