A soft sleeper? Soft sleepers are 4 beds in one cabin I believe.

Hard sleepers are 6 beds in one cabin.

Well, things you definitely need to bring:

1. water

2. toilet paper / tissues

3. mouthwash (not so convenient to brush there)

Other things I find which make my train rides easier:

1. mp3 player (if you plan on using ur phone, buy an extra battery)

2. a good book (plus a flashlight for when they turn the lights off in the train)

3. peanuts, cashews, sunflower seeds. Basically any kind of snacks that'll keep me occupied.

Beer and one of those little insulated zipper-bags with ice to keep 'em cold.

Consider that someone may beat you to that bottom bunk, and you'll have to kindly remind them which bunk is actually theirs.  I've never taken a sleeper, but 9 times out of 10, someone is already sitting in my assigned seat when I board the train.

If it were me, I'd prepare some entertainment.  iPhone games, iPod, books, whatever.  I don't usually expect my Chinese train mates to allow much peace and quiet for any meaningful sleeping.  Good luck!

i took a train from qingdao to guangzhou before on a hard seat, 29 hours . Prepared my ipod, psp, movies and a good book. Also prepare a load of snacks from your local supermarket.It really helps.

tablet pc is a must (with a long battery life.

the other things you need to bring are already mentioned.

but if you have the bottom bunk be prepared for everyone else in the cabin to use it to sit on. even while you're sleeping.

For me, having...

Snacks

Tablet

Phone with internet sharing so your tablet or netbook can access the Net

Drinks

Tissue

Also, try to get there extra early, and scout out a way to be first in line to run up to the train.  People never sit in assigned seats on trains, so I would fight for my bunk before someone else tries to claim it.  Most will move off as long as you aren't late getting on the train and they know there are no good seats left.

If you do have someone who tries to sit on your bunk, just cross your fingers for a cutie, and not a dirty smelly coal miner.

Don't enter train without iPod. You will likely get tired of listening snoring, and endless, loud phone conversations anytime in the day.

I can be without water 8 hours on the train, butt....I won't travel without iPod, ever again! 

You have a soft sleeper, so it will be much better than they described.  Usually older men occupy soft sleepers.  My considerable experienced with soft sleeper is quiet (relatively) cabin with middle class people who pretty much mind their business.  The bathrooms are much better and you will probably even have a western toilet.  Not as many people sharing the bathrooms means you can brush your teeth and wash in peace (with hot water!).  The Soft sleeper cabin has it's benefits which is why I always try to book early to catch one on long distance journeys.

I have never used  the soft sleeper

we always use the hard one with 6 bunks

a lot of the trains have tv now  but I like to catch up on sleep or just look out the window

and make funny faces at the people at the stations

Remember not to take your phone to the toilet and they are locked when the train is in the station

top bunk is good to hide from the locals  and to keep to yourself

 the bottom bunk is where every body and their dog will come and sit to talk and put their feet all over your bed

what about the middle bunk you ask

this is at head height for most and you will have that constant smell of bad breath on the back of your neck as they try to get a closer look at you

some trains have power points in the corridors but not all

I like my train rides as long as it is not hard seat sitting

even the 6 hours from Harbin to Mudanjaing we get a bed

keep your travel stuff separate from your main bag that way you do not have to keep opening a suit case and you can keep your survival kit near you at all times

a good travel cup is important for hot water to have a coffee if you have milk / sugar

 take some with you 

make shore you carry small notes as change can be difficult sometimes if buying from the trolley 

if you want to be sociable take playing cards  and coins

and finally

ear plugs!!!!

I will leave that one as a surprise