Smartecosse
What I've generally found to be a good setup for most people - although as Paul says it's always down to personal preference - is as a short trigger pull as you can get from the rest point to the firing point and then a longer pull of 1mm to 2mm - again personal preference - from the firing point to the trigger stop. The latter setting allows the trigger spring to provide a stronger return, which should help you achieve a faster rate of fire.
I see you have a Racegun - if you have an older version with the dome switch then you may find it harder to walk the trigger anyway. I've always found leaf microswitches and Hall (E-Mag) or optical (IR3) sensors easier to use since they generally require less force to activate. OK, it's a tiny difference and might just be me, but it makes it easier to just sort of tremble your fingers.
As far as learning how to do it, jhalad is right about the finger exercises, that really does help, plus you can also actually practice walking the trigger whilst watching TV - I would recommend watching England football matches and trying to shoot any English players, you'll be up to 20bps by the end of the match!
A final tip, I found it easier to start learning the technique by holding the grip very loosely, hardly at all in fact, and then as my fingers got used to the rhythm and coordination required I gradually worked towards actually holding the grip thus giving a high ROF and a stable marker.
I used to walk the trigger and was starting to achieve decent sustained and smooth rates of fire, getting up to 16bps, but now I'm getting slower and slower for some reason
so I've switched back to my vibrating middle finger method.
Anyway, hope that's been some help to you.