1) As soon as you arrive in Beijing, make arrangements to get the train tickets. I don't recommend wasting time at the station yourself--get your hotel to do it for a small add-on fee, or if you are near one of the satellite ticket offices go there for a no-hassle experience. If you have accommodations set up already, email them with your requirements and see if they will buy tickets for you and what the "commission" will be. Tickets only go on sale 5 or 10 days beforehand, and sleepers are always subject to selling out, so if you will not be in Beijing for long, an advance purchase by somebody local is a good idea.

2) There is no guarantee that a particular T train on a particular date will be carrying the deluxe soft sleepers, and you won't find out for sure until the day before, which is really gambling if you are on a fixed schedule. I personally would select a Z train and upper + lower berths in a standard soft sleeper compartment. It is safe and comfortable. Normally anybody else travelling soft-sleeper and sharing your compartment will be a solid middle-class type Chinese citizen or official, or perhaps other foreigners. Check out this thread for more detailed info, and in particular note the comments by poster "Ellyse", who is a friend of mine and one of the most experienced Chinese-train travellers I know, particularly the BJ-Shanghai route.

3) Especially in the last half of November and December, Beijing airport is subject to significant delays from fog. If you happen to be scheduled for air travel on a day that turns out to be foggy, you could go nowhere fast. The trains will run regardless, so stick with this mode of transport.