You will not only want to read the 8051 bible as suggested by Erik, but also read the datasheet for your part--sometimes various derivatives do things slightly different.

You will be looking for setting either the IDL or PD bit in PCON. Idle mode can be useful since a interrupt can wake the chip back up. In power down mode a hardware reset is required to get running again, so you won't be continuing from where you were.

Dennis