As long as it's not a "Golden Week" (Spring Festival, May Day and National Day), you shouldn't worry about getting a seat. Have your hotel book you the ticket, which should cost a small fee (say 30RMB). Soft sleeper is quite nice, depending on the route; I've found Shanghai-Beijing to be one of the best. It's also quite reasonable - I think Guilin-Beijing is something like 1200RMB, and the route passes through some nice scenery (and industrial areas, which I find interesting). The one advantage of hard sleeper is that, if you speak Chinese, you'll have a good opportunity to chat with the local Chinese, which certainly makes a 24-hour train ride pass by quicker!
The one thing you want to avoid is hard seat (or 'wuzuo' - no seat, which is exactly what it sounds like). Trust me. I did Nanning-Wuhan in wuzuo, and few days have been worse!