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. ;-)
You can get Xper points quicker with promoted myTakes (+50) and featured myTakes (+150, 200 total). If you're an editor I think all of your myTakes get promoted but even then to reach 50000 points you would have to write between 1000 myTakes or 250 if they're all featured.