Hello! It is simply, but you have to use CMOS version of 8051 (89C51, 89C51...)
Look at PCON register description. There are two bits PD & IDL. Setting PD (bit no. 1) bit causes Power Down operation, setting IDL (bit no.0) - Idle Mode. You have to decide, which mode you want to use. Once again: look at microprocessor description!
The register is not bit addressable. So you have to set theese bits by ORL command, f.e.
ORL PCON,#00000001B ;power down
But details are depending on your application! Good luck,
Jacek