Spray it down with water really well and give it lots of light. I've not had much luck at getting moss to actually *grow* more other than java moss. It seems to always be how long can I keep the pillow/whatever moss looking good before I replace it.
My dried spaghnum moss will turn green again after a few months of constant moisture and light. I'm not sure if it's the original dried ones that are coming back or if there are surviving spores that germinate in the old moss.
Well not too strong lighting, i use two 13 watt sun lights. The screwin cf's.
Also about the spaghum, I think its just a local company, but it might not be. BetterGro LongFiber Spaghum Moss. Not the orchid moss, it doesnt seem to have nearly the quality of the long fiber and seems to dry out way faster. I have had it come back to life in the tray i keep the stuff extra i moistened for my orchids.
Oh yeah, And the moss I use I in tanks is sheet moss, I get it off cypress knees if that helps at all.