theres a few methods. (courtesy of scott kelby).
a. use lightness channel - under image menu- mode - lab color & convert RGB photo into lab color mode. then go to channels palette and click on lightness palette.. under image menu, under mode, chose graysclale.
b. use channel mixture. from create new adjustment layer. turn on monochrome checkbox at the bottm dialog.
c. scotts mthod - press D to set foreground colour to black, then from create new adjustment layer, choose gradient map. click ok to apply it to photo. then go to layers palette, chose channel mixer. use this to adjust.
d. calculations method - open the color photo. under image menu, choose calculations. then adjust the channels and blend modes using the pop up menus. also, change blending pop-up menu from multiply to overlay... (can adjust further using opacity setting)