I have found that the black rosin is more effective than the amber type. Also definately 'scuff up' a new shiny block.

A local violin dealer/repairer enlightened me as to why bows actually need rehairing from time to time - the hair apparently has lots of hook-like structures (like this: JJJJJJJJ) . When you rosin the bow, the curved end of the hooks retain the rosin. As the bow is used through time, the hooked ends get worn away, so you are left with fine straight strands like this: |||||||||||. Once this happens, you can rub and rub the bow with rosin but it won't stick - time for a re-hair!