I find Manitou Prep-M grease to be the best. In terms of re-greasing intervals: I am sure different people will give you different answers, plus it depends on what conditions you ride in. I would say inject new grease in every 4-5 rides, in dirty/dusty conditions, but then again that I prefer to be on the safe side.
There is a guide on how to do it here http://www.mythic-bikes.co.uk/tech.htm
There is a guide on how to do it here http://www.mythic-bikes.co.uk/tech.htm