I personally don't shave my chest, but I don't see how much different it would be from shaving anywhere else.
1) Take a quick, very warm shower (5 min.) - this should steam up the skin pretty well. When showering, use an exfoliating scrub, a washcloth works but some sort of cloth specifically made for exfoliating the skin is best.
2) Use a razor bump reducing shaving lotion. Aveeno makes a great one, and I've heard good things about Neutrogena's (which is designed specifically for men).
3) Use the Mach3 Turbo. No other razor works as well as the Mach3, and the Turbo is the better version of the two.
4) Generally try to shave in the direction of the hair follicle (shaving against the grain will cause the bumps, but shaving against the grain is also the only way to get a truly smooth shave).
5) Follow shaving with a moisturizer. If you want, Aveeno also makes a shave-reducing moisturizer that is very nice. It will retard hair growth (you have to use this every day to notice the slower hair growth--plus it makes shaving easier the next time). Also, using an AHA lotion helps prevent the bumps. Maybe at night apply an AHA lotion, and in the morning, use the Aveeno stuff.
6) Tend Skin makes a razor bump lotion that is supposedly a miracle worker. Expensive as hell, but everyone says it works. It's basically a BHA with antiseptic qualities.
Alternatively, using an electric shaver will eliminate 99% of all razor bumps, if you go that route. It doesn't provide as smooth a shave, but if you aren't worried about people feeling a stubbly chest, then it will prevent most of the problem.