Most Helpful Girl
You just remain active and answer/ask questions. The xper adds up and will eventually give you a title when you reach a certain level of points.
Most Helpful Guy
I have been a member for quite some time now, and the best way is through posting and answering a lot of questions. I tend to answer more than post, so it has taken me quite a while to accumulate the points I have, but fortunately I'm a pretty patient person. ;-)0