When you arrive at Hong Kong Station (Airport Express) from HKG, you will need money. There’s a Citibank on the first mall floor (IFC 1) which is in the same building. Get your money there. If you have an HSBC account in the states, there’s ATMs in the arrivals hall near 7-11.
Leaving…If you have a morning flight, splurge on the cab ride to the AeX station. It’s only about HK$40 (US$5.00, give or take) and it will save you a lot of effort. Traffic doesn’t really begin to suck until 0800 or so, if you’re earlier that should be fine.
In-Town check in: the greatest boon to travellers since frequent flyer points. Use it. Then you can putter around the airport, relatively stress free, once you get to your terminal building. Speaking of that:
Once you get to the Airport, make all due haste to get to the correct building. If your gate is a higher numbered one, you’ll have to take a shuttle and walk a bit. Maybe about 10 minutes added to your journey.
Just some things to keep in mind should you arrive or depart all flustered and confused.